# Service-based model

![Perfetto Stack](https://storage.googleapis.com/perfetto/markdown_img/producer-service-consumer.png)

## Service

The tracing service is a long-lived entity (a system daemon on Linux/Android,
a service in Chrome) that has the following responsibilities:

* Maintains a registry of active producers and their data sources.
* Owns the trace buffers.
* Handles multiplexing of several tracing sessions.
* Routes the trace config from the consumers to the corresponding producers.
* Tells the Producers when and what to trace.
* Moves data from the Producer's shared memory buffer to the central non-shared
  trace buffers.

## Producer

A producer is an untrusted entity that offers the ability to contribute to the
trace. In a multiprocess model, a producer almost always corresponds to a client
process of the tracing service. It advertises its ability to contribute to the trace with one or more data sources.
Each producer has exactly:

* One shared memory buffer, shared exclusively with the tracing service.
* One IPC channel with the tracing service.

A producer is completely decoupled (both technically and conceptually) from
consumer(s). A producer knows nothing about:

* How many consumer(s) are connected to the service.
* How many tracing sessions are active.
* How many other producer(s) are registered or active.
* Trace data written by other producer(s).

NOTE: In rare circumstances a process can host more than one producer and hence more
than one shared memory buffer. This can be the case for a process bundling
third-party libraries that in turn include the Perfetto client library.  
Concrete example: at some point in the future Chrome might expose one Producer for tracing within the main project, one for V8 and one for Skia (for each child
process).

## Consumer
A consumer is a trusted entity (a cmdline client on Linux/Android, an interface
of the Browser process in Chrome) that controls (non-exclusively) the tracing service and reads back (destructively) the trace buffers.
A consumer has the ability to:
* Send a [trace config](#) to the service, determining:
 * How many trace buffers to create.
 * How big the trace buffers should be.
 * The policy for each buffer (*ring-buffer* or *stop-when-full*).
 * Which data sources to enable.
 * The configuration for each data source.
 * The target buffer for the data produced by each data source configured.
* Enable and disable tracing.
* Read back the trace buffers:
  * Streaming data over the IPC channel.
  * Passing a file descriptor to the service and instructing it to periodically
    save the trace buffers into the file.

## Data source

A data source is a capability, exposed by a Producer, of providing some tracing
data. A data source almost always defines its own schema (a protobuf) consisting
of:
* At most one `DataSourceConfig` sub-message:

  ([example](/protos/perfetto/config/ftrace/ftrace_config.proto))
* One or more `TracePacket` sub-messages
  ([example](/protos/perfetto/trace/ps/process_tree.proto))

Different producers may expose the same data source. A concrete example is
the case of processes using the
[Track Event in the Tracing SDK](/docs/instrumentation/track-events). That
exposes the same `track_event` data source in every process involved.


## IPC channel
In a multiprocess scenario, each producer and each consumer interact with the
service using an IPC channel. IPC is used only in non-fast-path interactions,
mostly handshakes such as enabling/disabling trace (consumer), (un)registering
and starting/stopping data sources (producer). The IPC is typically NOT employed
to transport the protobufs for the trace.
Perfetto provides a POSIX-friendly IPC implementation, based on protobufs over a
UNIX socket (see
[Socket protocol](/docs/design-docs/api-and-abi#socket-protocol)).

That IPC implementation is not mandated. Perfetto allows the embedder:

* Wrap its own IPC subsystem (e.g., Perfetto in Chromium uses Mojo)
* Not use an IPC mechanism at all and just short circuit the
  Producer <> Service <> Consumer interaction via `PostTask(s)`.

## Shared memory buffer
Producer(s) write tracing data, in the form of protobuf-encoded binary blobs,
directly into its shared memory buffer, using a special library called
[ProtoZero](/docs/design-docs/protozero.md). The shared memory buffer:

* Has a fixed and typically small size (configurable, default: 128 KB).
* Is an ABI and must maintain backwards compatibility.
* Is shared by all data sources of the producer.
* Is independent of the number and the size of the trace buffers.
* Is independent of the number of Consumer(s).
* Is partitioned in *chunks* of variable size.

Each chunk:

* Is owned exclusively by one Producer thread (or shared through a mutex).
* Contains a linear sequence of `TracePacket(s)`, or
  fragments of that. A `TracePacket` can span across several chunks, the
  fragmentation is not exposed to the consumers (consumers always see whole
  packets as if they were never fragmented).
* Can be owned and written by exactly one `TraceWriter`.
* Is part of a reliable and ordered sequence, identified by the `WriterID`:
  packets in a sequence are guaranteed to be read back in order, without gaps
  and without repetitions.

See the comments in
[shared_memory_abi.h](/include/perfetto/ext/tracing/core/shared_memory_abi.h)
for more details about the binary format of this buffer.
