Thrift whitepaper draft
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+%-----------------------------------------------------------------------------
+%
+% Thrift whitepaper
+%
+% Name: thrift.tex
+%
+% Authors: Mark Slee (mcslee@facebook.com)
+%
+% Created: 05 March 2007
+%
+%-----------------------------------------------------------------------------
+
+
+\documentclass[nocopyrightspace,blockstyle]{sigplanconf}
+
+\usepackage{amssymb}
+\usepackage{amsfonts}
+\usepackage{amsmath}
+
+\begin{document}
+
+% \conferenceinfo{WXYZ '05}{date, City.}
+% \copyrightyear{2007}
+% \copyrightdata{[to be supplied]}
+
+% \titlebanner{banner above paper title} % These are ignored unless
+% \preprintfooter{short description of paper} % 'preprint' option specified.
+
+\title{Thrift: Scalable Cross-Language Services Implementation}
+\subtitle{}
+
+\authorinfo{Mark Slee, Aditya Agarwal and Marc Kwiatkowski}
+ {Facebook, 156 University Ave, Palo Alto, CA}
+ {\{mcslee,aditya,marc\}@facebook.com}
+
+\maketitle
+
+\begin{abstract}
+Thrift is a software library and set of code-generation tools developed at
+Facebook to expedite development and implementation of efficient and scalable
+backend services. Its primary goal is to enable efficient and reliable
+communication across programming languages by abstracting the portions of each
+language that tend to require the most customization into a common library
+that is implemented in each language. Specifically, Thrift allows developers to
+define data types and service interfaces in a single language-neutral file
+and generate all the necessary code to build RPC clients and servers.
+
+This paper details the motivations and design choices we made in Thrift, as
+well as some of the more interesting implementation details. It is not
+intended to be taken as research, but rather it is an exposition on what we did
+and why.
+\end{abstract}
+
+% \category{D.3.3}{Programming Languages}{Language constructs and features}
+
+%\terms
+%Languages, serialization, remote procedure call
+
+%\keywords
+%Data description language, interface definition language, remote procedure call
+
+\section{Introduction}
+As Facebook's traffic and network structure have scaled, the resource
+demands of many operations on the site (i.e. search,
+ad selection and delivery, event logging) have presented technical requirements
+drastically outside the scope of the LAMP framework. In our implementation of
+these services, various programming languages have been selected to
+optimize for the right combination of performance, ease and speed of
+development, availability of existing libraries, etc. By and large,
+Facebook's engineering culture has tended towards choosing the best
+tools and implementations avaiable over standardizing on any one
+programming language and begrudgingly accepting its inherent limitations.
+
+Given this design choice, we were presented with the challenge of building
+a transparent, high-performance bridge across many programming languages.
+We found that most available solutions were either too limited, did not offer
+sufficient data type freedom, or suffered from subpar performance.
+\footnote{See Appendix A for a discussion of alternative systems.}
+
+The solution that we have implemented combines a language-neutral software
+stack implemented across numerous programming languages and an associated code
+generation engine that transforms a simple interface and data definition
+language into client and server remote procedure call libraries.
+Choosing static code generation over a dynamic system allows us to create
+validated code with implicit guarantees that can be run without the need for
+any advanced intropsecive run-time type checking. It is also designed to
+be as simple as possible for the developer, who can typically define all
+the necessary data structures and interfaces for a complex service in a single
+short file.
+
+Surprised that a robust open solution to these relatively common problems
+did not yet exist, we committed early on to making the Thrift implementation
+open source.
+
+In evaluating the challenges of cross-language interaction in a networked
+environment, some key components were identified:
+
+\textit{Types.} A common type system must exist across programming languages
+without requiring that the application developer use custom Thrift data types
+or write their own serialization code. That is,
+a C++ programmer should be able to transparently exchange a strongly typed
+STL map for a dynamic Python dictionary. Neither
+programmer should be forced to write any code below the application layer
+to achieve this. Section 2 details the Thrift type system.
+
+\textit{Transport.} Each language must have a common interface to
+bidirectional raw data transport. The specifics of how a given
+transport is implemented should not matter to the service developer.
+The same application code should be able to run against TCP stream sockets,
+raw data in memory, or files on disk. Section 3 details the Thrift Transport
+layer.
+
+\textit{Protocol.} Data types must have some way of using the Transport
+layer to encode and decode themselves. Again, the application
+developer need not be concerned by this layer. Whether the service uses
+an XML or binary protocol is immaterial to the application code.
+All that matters is that the data can be read and written in a consistent,
+deterministic matter. Section 4 details the Thrift Protocol layer.
+
+\textit{Versioning.} For robust services, the involved data types must
+provide a mechanism for versioning themselves. Specifically,
+it should be possible to add or remove fields in an object or alter the
+argument list of a function without any interruption in service (or,
+worse yet, nasty segmentation faults). Section 5 details Thrift's versioning
+system.
+
+\textit{Processors.} Finally, we generate code capable of processing data
+streams to accomplish remote procedure call. Section 6 details the generated
+code and TProcessor paradigm.
+
+Section 7 discusses implementation details, and Section 8 describes
+our conclusions.
+
+\section{Types}
+
+The goal of the Thrift type system is to enable programmers to develop using
+completely natively defined types, no matter what programming language they
+use. By design, the Thrift type system does not introduce any special dynamic
+types or wrapper objects. It also does not require that the developer write
+any code for object serialization or transport. The Thrift IDL file is
+logically a way for developers to annotate their data structures with the
+minimal amount of extra information necessary to tell a code generator
+how to safely transport the objects across languages.
+
+\subsection{Base Types}
+
+The type system rests upon a few base types. In considering which types to
+support, we aimed for clarity and simplicity over abundance, focusing
+on the key types available in all programming languages, ommitting any
+niche types available only in specific languages.
+
+The base types supported by Thrift are:
+\begin{itemize}
+\item \texttt{bool} A boolean value, true or false
+\item \texttt{byte} A signed byte
+\item \texttt{i16} A 16-bit signed integer
+\item \texttt{i32} A 32-bit signed integer
+\item \texttt{i64} A 64-bit signed integer
+\item \texttt{double} A 64-bit floating point number
+\item \texttt{string} An encoding-agnostic text or binary string
+\end{itemize}
+
+Of particular note is the absence of unsigned integer types. Because these
+types have no direct translation to native primitive types in many languages,
+the advantages they afford are lost. Further, there is no way to prevent the
+application developer in a language like Python from assigning a negative value
+to an integer variable, leading to unpredictable behavior. From a design
+standpoint, we observed that unsigned integers were very rarely, if ever, used
+for arithmetic purposes, but in practice were much more often used as keys or
+identifiers. In this case, the sign is irrelevant. Signed integers serve this
+same purpose and can be safely cast to their unsigned counterparts (most
+commonly in C++) when absolutely necessary.
+
+\subsection{Containers}
+
+Thrift containers are strongly typed containers that map to the most commonly
+used containers in common programming languages. They are annotated using
+C++ template (or Java Generics) style. There are three types available:
+\begin{itemize}
+\item \texttt{list<type>} An ordered list of elements. Translates directly into
+an STL vector, Java ArrayList, or native array in scripting languages. May
+contain duplicates.
+\item \texttt{set<type>} An unordered set of unique elements. Translates into
+an STL set, Java HashSet, or native dictionary in PHP/Python/Ruby.
+\item \texttt{map<type1,type2>} A map of strictly unique keys to values
+Translates into an STL map, Java HashMap, PHP associative array,
+or Python/Ruby dictionary.
+\end{itemize}
+
+While defaults are provided, the type mappings are not explicitly fixed. Custom
+code generator directives have been added to substitute custom types in
+destination languages (i.e.
+\texttt{hash\_map}, or Google's sparse hash map can be used in C++). The
+only requirement is that the custom types support all the necessary iteration
+primitives. Container elements may be of any valid Thrift type, including other
+containers or structs.
+
+\subsection{Structs}
+
+A Thrift struct defines a common objects to be used across languages. A struct
+is essentially equivalent to a class in object oriented programming
+languages. A struct has a set of strongly typed fields, each with a unique
+name identifier. The basic syntax for defining a Thrift struct looks very
+similar to a C struct definition. Fields may be annotated with an integer field
+identifier (unique to the scope of that struct) and optional default values.
+Field identifiers will be automatically assigned if omitted, though they are
+strongly encouraged for versioning reasons discussed later.
+
+\begin{verbatim}
+struct Example {
+ 1:i32 number=10,
+ 2:i64 bigNumber,
+ 3:double decimals,
+ 4:string name="thrifty"
+}\end{verbatim}
+
+In the target language, each definition generates a type with two methods,
+\texttt{read} and \texttt{write}, which perform serialization and transport
+of the objects using a Thrift TProtocol object.
+
+\subsection{Exceptions}
+
+Exceptions are syntactically and functionally equivalent to structs except
+that they are declared using the \texttt{exception} keyword instead of the
+\texttt{struct} keyword.
+
+The generated objects inherit from an exception base class as appropriate
+in each target programming language, the goal being to offer seamless
+integration with native exception handling for the developer in any given
+language. Again, the design emphasis is on making the code familiar to the
+application developer.
+
+\subsection{Services}
+
+Services are defined using Thrift types. Definition of a service is
+semantically equivalent to defining a pure virtual interface in object oriented
+programming. The Thrift compiler generates fully functional client and
+server stubs that implement the interface. Services are defined as follows:
+
+\begin{verbatim}
+service <name> {
+ <returntype> <name>(<arguments>)
+ [throws (<exceptions>)]
+ ...
+}\end{verbatim}
+
+An example:
+
+\begin{verbatim}
+service StringCache {
+ void set(1:i32 key, 2:string value),
+ string get(1:i32 key) throws (1:KeyNotFound knf),
+ void delete(1:i32 key)
+}
+\end{verbatim}
+
+Note that \texttt{void} is a valid type for a function return, in addition to
+all other defined Thrift types. Additionally, an \texttt{async} modifier
+keyword may be added to a void function, which will generate code that does
+not wait for a response from the server. Note that a pure \texttt{void}
+function will return a response to the client which guarantees that the
+operation has completed on the server side. With \texttt{async} method calls
+the client can only be guaranteed that the request succeeded at the
+transport layer. (In many transport scenarios this is inherently unreliable
+due to the Byzantine Generals' Problem. Therefore, application developers
+should take care only to use the async optimization in cases where dopped
+method calls are acceptable or the transport is known to be reliable.)
+
+Also of note is the fact that argument and exception lists to functions are
+implemented as Thrift structs. They are identical in both notation and
+behavior.
+
+\section{Transport}
+
+The transport layer is used by the generated code to facilitate data transfer.
+
+\subsection{Interface}
+
+A key design choice in the implementation of Thrift was to abstract the
+transport layer from the code generation layer. Though Thrift is typically
+used on top of the TCP/IP stack with streaming sockets as the base layer of
+communication, there was no compelling reason to build that constraint into
+the system. The performance tradeoff incurred by an abstracted I/O layer
+(roughly one virtual method lookup / function call per operation) was
+immaterial compared to the cost of actual I/O operations (typically invoking
+system calls).
+
+Fundamentally, generated Thrift code just needs to know how to read and
+write data. Where the data is going is irrelevant, it may be a socket, a
+segment of shared memory, or a file on the local disk. The Thrift transport
+interface supports the following methods.
+
+\begin{itemize}
+\item \texttt{open()} Opens the tranpsort
+\item \texttt{close()} Closes the tranport
+\item \texttt{isOpen()} Whether the transport is open
+\item \texttt{read()} Reads from the transport
+\item \texttt{write()} Writes to the transport
+\item \texttt{flush()} Force any pending writes
+\end{itemize}
+
+There are a few additional methods not documented here which are used to aid
+in batching reads and optionally signaling completion of reading or writing
+chunks of data by the generated code.
+
+In addition to the above
+\texttt{TTransport} interface, there is a \texttt{TServerTransport} interface
+used to accept or create primitive transport objects. Its interface is as
+follows:
+
+\begin{itemize}
+\item \texttt{open()} Opens the tranpsort
+\item \texttt{listen()} Begins listening for connections
+\item \texttt{accept()} Returns a new client transport
+\item \texttt{close()} Closes the transport
+
+\end{itemize}
+
+\subsection{Implementation}
+
+The transport interface is designed for simple implementation in any
+programming language. New transport mechanisms can be easily defined as needed
+by application developers.
+
+\subsubsection{TSocket}
+
+The \texttt{TSocket} class is implemented across all target languages. It
+provides a common, simple interface to a TCP/IP stream socket.
+
+\subsubsection{TFileTransport}
+
+The \texttt{TFileTransport} is an abstraction of an on-disk file to a data
+stream. It allows Thrift data structures to be used as historical log data.
+Essentially, an application developer can use a \texttt{TFileTransport} to
+write out a set of
+requests to a file on disk. Later, this data may be replayed from the log,
+either for post-processing or for recreation and simulation of previous events.
+
+\subsubsection{Utilities}
+
+The Transport interface is designed to support easy extension using common
+OOP techniques such as composition. Some simple utilites include the
+\texttt{TBufferedTransport}, which buffers writes and reads on an underlying
+transport, the \texttt{TFramedTransport}, which transmits data with frame
+size headers for chunking optimzation or nonblocking operation, and the
+\texttt{TMemoryBuffer}, which allows reading and writing directly from heap or
+stack memory owned by the process.
+
+\section{Protocol}
+
+A second major abstraction in Thrift is the separation of data structure from
+transport representation. Thrift enforces a certain messaging structure when
+transporting data, but it is agnostic to the protocol encoding in use. That is,
+it does not matter whether data is encoded in XML, human-readable ASCII, or a
+dense binary format, so long as the data supports a fixed set of operations
+that allow generated code to deterministically read and write.
+
+\subsection{Interface}
+
+The Thrift Protocol interface is very straightforward. It fundamentally
+supports two things: 1) bidirectional sequenced messaging, and
+2) encoding of base types, containers, and structs.
+
+\begin{verbatim}
+writeMessageBegin(name, type, seq)
+writeMessageEnd()
+writeStructBegin(name)
+writeStructEnd()
+writeFieldBegin(name, type, id)
+writeFieldEnd()
+writeFieldStop()
+writeMapBegin(ktype, vtype, size)
+writeMapEnd()
+writeListBegin(etype, size)
+writeListEnd()
+writeSetBegin(etype, size)
+writeSetEnd()
+writeBool(bool)
+writeByte(byte)
+writeI16(i16)
+writeI32(i32)
+writeI64(i64)
+writeDouble(double)
+writeString(string)
+
+name, type, seq = readMessageBegin()
+ readMessageEnd()
+name = readStructBegin()
+ readStructEnd()
+name, type, id = readFieldBegin()
+ readFieldEnd()
+k, v, size = readMapBegin()
+ readMapEnd()
+etype, size = readListBegin()
+ readListEnd()
+etype, size = readSetBegin()
+ readSetEnd()
+bool = readBool()
+byte = readByte()
+i16 = readI16()
+i32 = readI32()
+i64 = readI64()
+double = readDouble()
+string = readString()
+\end{verbatim}
+
+Note that every write function has exactly one read function counterpart, with
+the exception of the \texttt{writeFieldStop()} method. This is a special method
+that signals the end of a struct. The procedure for reading a struct is to
+\texttt{readFieldBegin()} until the stop field is encountered, and to then
+\texttt{readStructEnd()}. The
+generated code relies upon this structure to ensure that everything written by
+a protocol encoder can be read by a matching protocol decoder. Further note
+that this set of functions is by design more robust than necessary.
+For example, \texttt{writeStructEnd()} is not strictly necessary, as the end of
+a struct may be implied by the stop field. This method is a convenience for
+verbose protocols where it is cleaner to separate these calls (i.e. a closing
+\texttt{</struct>} tag in XML).
+
+\subsection{Structure}
+
+Thrift structures are designed to support encoding into a streaming
+protocol. That is, the implementation should never need to frame or compute the
+entire data length of a structure prior to encoding it. This is critical to
+performance in many scenarios. Consider a long list of relatively large
+strings. If the protocol interface required reading or writing a list as an
+atomic operation, then the implementation would require a linear pass over the
+entire list before encoding any data. However, if the list can be written
+as iteration is performed, the corresponding read may begin in parallel,
+theoretically offering an end-to-end speedup of $kN - C$, where $N$ is the size
+of the list, $k$ the cost factor associated with serializing a single
+element, and $C$ is fixed offset for the delay between data being written
+and becoming available to read.
+
+Similarly, structs do not encode their data lengths a priori. Instead, they are
+encoded as a sequence of fields, with each field having a type specifier and a
+unique field identifier. Note that the inclusion of type specifiers enables
+the protocol to be safely parsed and decoded without any generated code
+or access to the original IDL file. Structs are terminated by a field header
+with a special \texttt{STOP} type. Because all the basic types can be read
+deterministically, all structs (including those with nested structs) can be
+read deterministically. The Thrift protocol is self-delimiting without any
+framing and regardless of the encoding format.
+
+In situations where streaming is unnecessary or framing is advantageous, it
+can be very simply added into the transport layer, using the
+\texttt{TFramedTransport} abstraction.
+
+\subsection{Implementation}
+
+Facebook has implemented and deployed a space-efficient binary protocol which
+is used by most backend services. Essentially, it writes all data
+in a flat binary format. Integer types are converted to network byte order,
+strings are prepended with their byte length, and all message and field headers
+are written using the primitive integer serialization constructs. String names
+for fields are omitted - when using generated code, field identifiers are
+sufficient.
+
+We decided against some extreme storage optimizations (i.e. packing
+small integers into ASCII or using a 7-bit continuation format) for the sake
+of simplicity and clarity in the code. These alterations can easily be made
+if and when we encounter a performance critical use case that demands them.
+
+\section{Versioning}
+
+Thrift is robust in the face of versioning and data definition changes. This
+is critical to enable a staged rollout of changes to deployed services. The
+system must be able to support reading of old data from logfiles, as well as
+requests from out of date clients to new servers, or vice versa.
+
+\subsection{Field Identifiers}
+
+Versioning in Thrift is implemented via field identifiers. The field header
+for every member of a struct in Thrift is encoded with a unique field
+identifier. The combination of this field identifier and its type specifier
+is used to uniquely identify the field. The Thrift definition language
+supports automatic assignment of field identifiers, but it is good
+programming practice to always explicitly specify field identifiers.
+Identifiers are specified as follows:
+
+\begin{verbatim}
+struct Example {
+ 1:i32 number=10,
+ 2:i64 bigNumber,
+ 3:double decimals,
+ 4:string name="thrifty"
+}\end{verbatim}
+
+To avoid conflicts, fields with omitted identifiers are automatically assigned
+decrementing from -1, and the language only supports the manual assignment of
+positive identifiers.
+
+When data is being deserialized, the generated code can use these identifiers
+to properly identify the field and determine whether it aligns with a field in
+its definition file. If a field identifier is not recognized, the generated
+code can use the type specifier to skip the unknown field without any error.
+Again, this is possible due to the fact that all data types are self
+delimiting.
+
+Field identifiers can (and should) also be specified in function argument
+lists. In fact, argument lists are not only represented as structs on the
+backend, but actually share the same code in the compiler frontend. This
+allows for version-safe modification of method parameters
+
+\begin{verbatim}
+service StringCache {
+ void set(1:i32 key, 2:string value),
+ string get(1:i32 key) throws (1:KeyNotFound knf),
+ void delete(1:i32 key)
+}
+\end{verbatim}
+
+The syntax for specifying field identifiers was chosen to echo their structure.
+Structs can be thought of as a dictionary where the identifiers are keys, and
+the values are strongly typed, named fields.
+
+Field identifiers internally use the \texttt{i16} Thrift type. Note, however,
+that the \texttt{TProtocol} abstraction may encode identifiers in any format.
+
+\subsection{Isset}
+
+When an unexpected field is encountered, it can be safely ignored and
+discarded. When an expected field is not found, there must be some way to
+signal to the developer that it was not present. This is implemented via an
+inner \texttt{isset} structure inside the defined objects. (In PHP, this is
+implicit with a \texttt{null} value, or \texttt{None} in Python
+and \texttt{nil} in Ruby.) Essentially,
+the inner \texttt{isset} object of each Thrift struct contains a boolean value
+for each field which denotes whether or not that field is present in the
+struct. When a reader receives a struct, it should check for a field being set
+before operating directly on it.
+
+\begin{verbatim}
+class Example {
+ public:
+ Example() :
+ number(10),
+ bigNumber(0),
+ decimals(0),
+ name("thrifty") {}
+
+ int32_t number;
+ int64_t bigNumber;
+ double decimals;
+ std::string name;
+
+ struct __isset {
+ __isset() :
+ number(false),
+ bigNumber(false),
+ decimals(false),
+ name(false) {}
+ bool number;
+ bool bigNumber;
+ bool decimals;
+ bool name;
+ } __isset;
+...
+}
+\end{verbatim}
+
+\subsection{Case Analysis}
+
+There are four cases in which version mismatches may occur.
+
+\begin{enumerate}
+\item \textit{Added field, old client, new server.} In this case, the old
+client does not send the new field. The new server recognizes that the field
+is not set, and implements default behavior for out of date requests.
+\item \textit{Removed field, old client, new server.} In this case, the old
+client sends the removed field. The new server simply ignores it.
+\item \textit{Added field, new client, old server.} The new client sends a
+field that the old server does not recognize. The old server simply ignores
+it and processes as normal.
+\item \textit{Removed field, new client, old server.} This is the most
+dangerous case, as the old server is unlikely to have suitable default
+behavior implemented for the missing field. It is recommended that in this
+situation the new server be rolled out prior to the new clients.
+\end{enumerate}
+
+\subsection{Protocol/Transport Versioning}
+The \texttt{TProtocol} abstractions are also designed to give protocol
+implementations the freedom to version themselves in whatever manner they
+see fit. Specifically, any protocol implementation is free to send whatever
+it likes in the \texttt{writeMessageBegin()} call. It is entirely up to the
+implementor how to handle versioning at the protocol level. The key point is
+that protocol encoding changes are safely isolated from interface definition
+version changes.
+
+Note that the exact same is true of the \texttt{TTransport} interface. For
+example, if we wished to add some new checksumming or error detection to the
+\texttt{TFileTransport}, we could simply add a version header into the
+data it writes to the file in such a way that it would still accept old
+logfiles without the given header.
+
+\section{RPC Implementation}
+
+\subsection{TProcessor}
+
+The last core interface in the Thrift design is the \texttt{TProcessor},
+perhaps the most simple of the constructs. The interface is as follows:
+
+\begin{verbatim}
+interface TProcessor {
+ bool process(TProtocol in, TProtocol out)
+ throws TException
+}
+\end{verbatim}
+
+The key design idea here is that the complex systems we build can fundamentally
+be broken down into agents or services that operate on inputs and outputs. In
+most cases, there is actually just one input and output (an RPC client) that
+needs handling.
+
+\subsection{Generated Code}
+
+When a service is defined, we generate a
+\texttt{TProcessor} instance capable of handling RPC requests to that service,
+using a few helpers. The fundamental structure (illustrated in pseudo-C++) is
+as follows:
+
+\begin{verbatim}
+Service.thrift
+ => Service.cpp
+ interface ServiceIf
+ class ServiceClient : virtual ServiceIf
+ TProtocol in
+ TProtocol out
+ class ServiceProcessor : TProcessor
+ ServiceIf handler
+
+ServiceHandler.cpp
+ class ServiceHandler : virtual ServiceIf
+
+TServer.cpp
+ TServer(TProcessor processor,
+ TServerTransport transport,
+ TTransportFactory tfactory,
+ TProtocolFactory pfactory)
+ serve()
+\end{verbatim}
+
+From the thrift definition file, we generate the virtual service interface.
+A client class is generated, which implements the interface and
+uses two \texttt{TProtocol} instances to perform the I/O operations. The
+generated processor implements the \texttt{TProcessor} interface. The generated
+code has all the logic to handle RPC invocations via the \texttt{process()}
+call, and takes as a parameter an instance of the service interface,
+implemented by the application developer.
+
+The user provides an implementation of the application interface in their own,
+non-generated source file.
+
+\subsection{TServer}
+
+Finally, the Thrift core libraries provide a \texttt{TServer} abstraction.
+The \texttt{TServer} object generally works as follows.
+
+\begin{itemize}
+\item Use the \texttt{TServerTransport} to get a \texttt{TTransport}
+\item Use the \texttt{TTransportFactory} to optionally convert the primitive
+transport into a suitable application transport (typically the
+\texttt{TBufferedTransportFactory} is used here)
+\item Use the \texttt{TProtocolFactory} to create an input and output protocol
+for the \texttt{TTransport}
+\item Invoke the \texttt{process()} method of the \texttt{TProcessor} object
+\end{itemize}
+
+The layers are appropriately separated such that the server code needs to know
+nothing about any of the transports, encodings, or applications in play. The
+server encapsulates the logic around connection handling, threading, etc.
+while the processor deals with RPC. The only code written by the application
+developer lives in the definitional thrift file and the interface
+implementation.
+
+Facebook has deployed multiple \texttt{TServer} implementations, including
+the single-threaded \texttt{TSimpleServer}, thread-per-connection
+\texttt{TThreadedServer}, and thread-pooling \texttt{TThreadPoolServer}.
+
+The \texttt{TProcessor} interface is very general by design. There is no
+requirement that a \texttt{TServer} take a generated \texttt{TProcessor}
+object. Thrift allows the application developer to easily write any type of
+server that operates on \texttt{TProtocol} objects (for instance, a server
+could simply stream a certain type of object without any actual RPC method
+invocation).
+
+\section{Implementation Details}
+\subsection{Target Languages}
+Thrift currently supports five target languages: C++, Java, Python, Ruby, and
+PHP. At Facebook, we have deployed servers predominantly in C++, Java, and
+Python. Thrift services implemented in PHP have also been embedded into the
+Apache web server, providing transparent backend access to many of our
+frontend constructs using a \texttt{THttpClient} implementation of the
+\texttt{TTransport} interface.
+
+Though Thrift was explicitly designed to be much more efficient and robust
+than typical web technologies, as we were designing our XML-based REST web
+services API we noticed that Thrift could be easily used to define our
+service interface. Though we do not currently employ SOAP envelopes (in the
+author's opinion there is already far too much repetetive enterprise Java
+software to do that sort of thing), we were able to quickly extend Thrift to
+generate XML Schema Definition files for our service, as well as a framework
+for versioning different implementations of our web service. Though public
+web services are admittedly tangential to Thrift's core use case and design,
+Thrift facilitated rapid iteration and affords us the ability to quickly
+migrate our entire XML-based web service onto a higher performance system
+should the future need arise.
+
+\subsection{Generated Structs}
+We made a conscious decision to make our generated structs as transparent as
+possible. All fields are publicly accessible; there are no \texttt{set()} and
+\texttt{get()} methods. Similarly, use of the \texttt{isset} object is not
+enforced. We do not include any \texttt{FieldNotSetException} construct.
+Developers have the option to use these fields to write more robust code, but
+the system is robust to the developer ignoring the \texttt{isset} construct
+entirely and will provide suitable default behavior in all cases.
+
+The reason for this choice was for ease of application development. Our stated
+goal is not to make developers learn a rich new library in their language of
+choice, but rather to generate code that allow them to work with the constructs
+that are most familiar in each language.
+
+We also made the \texttt{read()} and \texttt{write()} methods of the generated
+objects public members so that the objects can be used outside of the context
+of RPC clients and servers. Thrift is a useful tool simply for generating
+objects that are easily serializable across programming languages.
+
+\subsection{RPC Method Identification}
+Method calls in RPC are implemented by sending the method name as a string. One
+issue with this approach is that longer method names require more bandwidth.
+We experimented with using fixed-size hashes to identify methods, but in the
+end concluded that the savings were not worth the headaches incurred. Reliably
+dealing with conflicts across versions of an interface definition file is
+impossible without a meta-storage system (i.e. to generate non-conflicting
+hashes for the current version of a file, we would have to know about all
+conflicts that ever existed in any previous version of the file).
+
+We wanted to avoid too many unnecessary string comparisons upon
+method invocation. To deal with this, we generate maps from strings to function
+pointers, so that invocation is effectively accomplished via a constant-time
+hash lookup in the common case. This requires the use of a couple interesting
+code constructs. Because Java does not have function pointers, process
+functions are all private member classes implementing a common interface.
+
+\begin{verbatim}
+private class ping implements ProcessFunction {
+ public void process(int seqid,
+ TProtocol iprot,
+ TProtocol oprot)
+ throws TException
+ { ...}
+}
+
+HashMap<String,ProcessFunction> processMap_ =
+ new HashMap<String,ProcessFunction>();
+\end{verbatim}
+
+In C++, we use a relatively esoteric language construct: member function
+pointers.
+
+\begin{verbatim}
+std::map<std::string,
+ void (ExampleServiceProcessor::*)(int32_t,
+ facebook::thrift::protocol::TProtocol*,
+ facebook::thrift::protocol::TProtocol*)>
+ processMap_;
+\end{verbatim}
+
+Using these techniques, the cost of string processing is minimized, and we
+reap the benefit of being able to easily debug corrupt or misunderstood data by
+looking for string contents.
+
+\subsection{Servers and Multithreading}
+MARC TO WRITE THIS SECTION ON THE C++ concurrency PACKAGE AND
+BASIC TThreadPoolServer PERFORMANCE ETC. (ie. 140K req/second, that kind of
+thing)
+
+\subsection{Nonblocking Operation}
+Though the Thrift transport interfaces map more directly to a blocking I/O
+model, we have implemented a high performance \texttt{TNonBlockingServer}
+in C++ based upon \texttt{libevent} and the \texttt{TFramedTransport}. We
+implemented this by moving all I/O into one tight event loop using a
+state machine. Essentially, the event loop reads framed requests into
+\texttt{TMemoryBuffer} objects. Once entire requests are ready, they are
+dispatched to the \texttt{TProcessor} object which can read directly from
+the data in memory.
+
+\subsection{Compiler}
+The Thrift compiler is implemented in C++ using standard lex/yacc style
+tokenization and parsing. Though it could have been implemented with fewer
+lines of code in another language (i.e. Python/PLY or ocamlyacc), using C++
+forces explicit definition of the language constructs. Strongly typing the
+parse tree elements (debatably) makes the code more approachable for new
+developers.
+
+Code generation is done using two passes. The first pass looks only for
+include files and type definitions. Type definitions are not checked during
+this phase, since they may depend upon include files. All included files
+are sequentially scanned in a first pass. Once the include tree has been
+resolved, a second pass is taken over all files which inserts type definitions
+into the parse tree and raises an error on any undefined types. The program is
+then generated against the parse tree.
+
+Due to inherent complexities and potential for circular dependencies,
+we explicitly disallow forward declaration. Two Thrift structs cannot
+each contain an instance of the other. (Since we do not allow \texttt{null}
+struct instances in the generated C++ code, this would actually be impossible.)
+
+\section{Conclusions}
+Thrift has enabled Facebook to build scalable backend
+services efficiently by enabling engineers to divide and conquer. Application
+developers can focus upon application code without worrying about the
+sockets layer. We avoid duplicated work by writing buffering and I/O logic
+in one place, rather than interspersing it in each application.
+
+Thrift has been employed in a wide variety of applications at Facebook,
+including search, logging, mobile, ads, and platform. We have
+found that the marginal performance cost incurred by an extra layer of
+software abstraction is eclipsed by the gains in developer efficiency and
+systems reliability.
+
+\appendix
+
+\section{Similar Systems}
+The following are software systems similar to Thrift. Each is (very!) briefly
+described:
+
+\begin{itemize}
+\item \textit{SOAP.} XML-based. Designed for web services via HTTP, excessive
+XML parsing overhead.
+\item \textit{CORBA.} Relatively comprehensive, debatably overdesigned and
+heavyweight. Comparably cumbersome software installation.
+\item \textit{COM.} Embraced mainly in Windows client softare. Not an entirely
+open solution.
+\item \textit{Pillar.} Lightweight and high-performance, but missing versioning
+and abstraction.
+\item \textit{Protocol Buffers.} Closed-source, owned by Google. Described in
+Sawzall paper.
+\end{itemize}
+
+\acks
+
+Many thanks for feedback on Thrift (and extreme trial by fire) are due to
+Martin Smith, Karl Voskuil, and Yishan Wong.
+
+Thrift is a successor to Pillar, a similar system developed
+by Adam D'Angelo, first while at Caltech and continued later at Facebook.
+Thrift simply would not have happened without Adam's insights.
+
+%\begin{thebibliography}{}
+
+%\bibitem{smith02}
+%Smith, P. Q. reference text
+
+%\end{thebibliography}
+
+\end{document}