docs/design-docs/api-and-abi.md

# Tracing API and ABI: surfaces and stability

This document describes the API and ABI surface of the
[Perfetto Client Library][cli_lib], what can be expected to be stable long-term
and what not.

#### In summary

* The public C++ API in `include/perfetto/tracing/` is mostly stable but can
  occasionally break at compile-time throughout 2020.
* The C++ API within `include/perfetto/ext/` is internal-only and exposed only
  for Chromium.
* A new C API/ABI for a tracing shared library is in the works in
  `include/perfetto/public`. It is not stable yet.
* The tracing protocol ABI is based on protobuf-over-UNIX-socket and shared
  memory. It is long-term stable and maintains compatibility in both directions
  (old service + newer client and vice-versa).
* The [DataSourceDescriptor][data_source_descriptor.proto],
  [DataSourceConfig][data_source_config.proto] and
  [TracePacket][trace-packet-ref] protos are updated maintaining backwards
  compatibility unless a message is marked as experimental. Trace Processor
  deals with importing older trace formats.
* There isn't a version number neither in the trace file nor in the tracing
  protocol and there will never be one. Feature flags are used when necessary.

## C++ API

The Client Library C++ API allows an app to contribute to the trace with custom
trace events. Its headers live under [`include/perfetto/`](/include/perfetto).

There are three different tiers of this API, offering increasingly higher
expressive power, at the cost of increased complexity. The three tiers are built
on top of each other. (Googlers, for more details see also
[go/perfetto-client-api](http://go/perfetto-client-api)).

![C++ API](/docs/images/api-and-abi.png)

### Track Event (public)

This mainly consists of the `TRACE_EVENT*` macros defined in
[`track_event.h`](/include/perfetto/tracing/track_event.h).
Those macros provide apps with a quick and easy way to add common types of
instrumentation points (slices, counters, instant events).
For details and instructions see the [Client Library doc][cli_lib].

### Custom Data Sources (public)

This consists of the `perfetto::DataSource` base class and the
`perfetto::Tracing` controller class defined in
[`tracing.h`](/include/perfetto/tracing.h).
These classes allow an app to create custom data sources which can get
notifications about tracing sessions lifecycle and emit custom protos in the
trace (e.g. memory snapshots, compositor layers, etc).

For details and instructions see the [Client Library doc][cli_lib].

Both the Track Event API and the custom data source are meant to be a public
API.

WARNING: The team is still iterating on this API surface. While we try to avoid
         deliberate breakages, some occasional compile-time breakages might be
         encountered when updating the library. The interface is expected to
         stabilize by the end of 2020.

### Producer / Consumer API (internal)

This consists of all the interfaces defined in the
[`include/perfetto/ext`](/include/perfetto/ext) directory. These provide access
to the lowest levels of the Perfetto internals (manually registering producers
and data sources, handling all IPCs).

These interfaces will always be highly unstable. We highly discourage
any project from depending on this API because it is too complex and extremely
hard to get right.
This API surface exists only for the Chromium project, which has unique
challenges (e.g., its own IPC system, complex sandboxing models) and has dozens
of subtle use cases accumulated through over ten years of legacy of
chrome://tracing. The team is continuously reshaping this surface to gradually
migrate all Chrome Tracing use cases over to Perfetto.

## Tracing Protocol ABI

The Tracing Protocol ABI consists of the following binary interfaces that allow
various processes in the operating system to contribute to tracing sessions and
inject tracing data into the tracing service:

 * [Socket protocol](#socket-protocol)
 * [Shared memory layout](#shmem-abi)
 * [Protobuf messages](#protos)

The whole tracing protocol ABI is binary stable across platforms and is updated
maintaining both backwards and forward compatibility. No breaking changes
have been introduced since its first revision in Android 9 (Pie, 2018).
See also the [ABI Stability](#abi-stability) section below.

![Tracing protocol](/docs/images/tracing-protocol.png)

### {#socket-protocol} Socket protocol

At the lowest level, the tracing protocol is initiated with a UNIX socket of
type `SOCK_STREAM` to the tracing service.
The tracing service listens on two distinct sockets: producer and consumer.

![Socket protocol](/docs/images/socket-protocol.png)

Both sockets use the same wire protocol, the `IPCFrame` message defined in
[wire_protocol.proto](/protos/perfetto/ipc/wire_protocol.proto). The wire
protocol is simply based on a sequence of length-prefixed messages of the form:
```
< 4 bytes len little-endian > < proto-encoded IPCFrame >

04 00 00 00 A0 A1 A2 A3   05 00 00 00 B0 B1 B2 B3 B4  ...
{ len: 4  } [ Frame 1 ]   { len: 5  } [   Frame 2  ]
```

The `IPCFrame` proto message defines a request/response protocol that is
compatible with the [protobuf services syntax][proto_rpc]. `IPCFrame` defines
the following frame types:

1. `BindService   {producer, consumer} -> service`<br>
    Binds to one of the two service ports (either `producer_port` or
    `consumer_port`).

2. `BindServiceReply  service -> {producer, consumer}`<br>
    Replies to the bind request, listing all the RPC methods available, together
    with their method ID.

3. `InvokeMethod   {producer, consumer} -> service`<br>
    Invokes a RPC method, identified by the ID returned by `BindServiceReply`.
    The invocation takes as unique argument a proto sub-message. Each method
    defines a pair of _request_ and _response_ method types.<br>
    For instance the `RegisterDataSource` defined in [producer_port.proto] takes
    a `perfetto.protos.RegisterDataSourceRequest` and returns a
    `perfetto.protos.RegisterDataSourceResponse`.

4. `InvokeMethodReply  service -> {producer, consumer}`<br>
    Returns the result of the corresponding invocation or an error flag.
    If a method return signature is marked as `stream` (e.g.
    `returns (stream GetAsyncCommandResponse)`), the method invocation can be
    followed by more than one `InvokeMethodReply`, all with the same
    `request_id`. All replies in the stream except for the last one will have
    `has_more: true`, to notify the client more responses for the same invocation
    will follow.

Here is how the traffic over the IPC socket looks like:

```
# [Prd > Svc] Bind request for the remote service named "producer_port"
request_id: 1
msg_bind_service { service_name: "producer_port" }

# [Svc > Prd] Service reply.
request_id: 1
msg_bind_service_reply: {
  success:    true
  service_id: 42
  methods:    {id: 2; name: "InitializeConnection" }
  methods:    {id: 5; name: "RegisterDataSource" }
  methods:    {id: 3; name: "UnregisterDataSource" }
  ...
}

# [Prd > Svc] Method invocation (RegisterDataSource)
request_id: 2
msg_invoke_method: {
  service_id: 42  # "producer_port"
  method_id:  5   # "RegisterDataSource"

  # Proto-encoded bytes for the RegisterDataSourceRequest message.
  args_proto: [XX XX XX XX]
}

# [Svc > Prd] Result of RegisterDataSource method invocation.
request_id: 2
msg_invoke_method_reply: {
  success:     true
  has_more:    false  # EOF for this request

  # Proto-encoded bytes for the RegisterDataSourceResponse message.
  reply_proto: [XX XX XX XX]
}
```

#### Producer socket

The producer socket exposes the RPC interface defined in [producer_port.proto].
It allows processes to advertise data sources and their capabilities, receive
notifications about the tracing session lifecycle (trace being started, stopped)
and signal trace data commits and flush requests.

This socket is also used by the producer and the service to exchange a
tmpfs file descriptor during initialization for setting up the
[shared memory buffer](/docs/concepts/buffers.md) where tracing data will be
written (asynchronously).

On Android this socket is linked at `/dev/socket/traced_producer`. On all
platforms it is overridable via the `PERFETTO_PRODUCER_SOCK_NAME` env var.

On Android all apps and most system processes can connect to it
(see [`perfetto_producer` in SELinux policies][selinux_producer]).

In the Perfetto codebase, the [`traced_probes`](/src/traced/probes/) and
[`heapprofd`](/src/profiling/memory) processes use the producer socket for
injecting system-wide tracing / profiling data.

#### Consumer socket

The consumer socket exposes the RPC interface defined in [consumer_port.proto].
The consumer socket allows processes to control tracing sessions (start / stop
tracing) and read back trace data.

On Android this socket is linked at `/dev/socket/traced_consumer`. On all
platforms it is overridable via the `PERFETTO_CONSUMER_SOCK_NAME` env var.

Trace data contains sensitive information that discloses the activity the
system (e.g., which processes / threads are running) and can allow side-channel
attacks. For this reason the consumer socket is intended to be exposed only to
a few privileged processes.

On Android, only the `adb shell` domain (used by various UI tools like
[Perfetto UI](https://ui.perfetto.dev/),
[Android Studio](https://developer.android.com/studio) or the
[Android GPU Inspector](https://github.com/google/agi))
and few other trusted system services are allowed to access the consumer socket
(see [traced_consumer in SELinux][selinux_consumer]).

In the Perfetto codebase, the [`perfetto`](/docs/reference/perfetto-cli)
binary (`/system/bin/perfetto` on Android) provides a consumer implementation
and exposes it through a command line interface.

#### Socket protocol FAQs

_Why SOCK_STREAM and not DGRAM/SEQPACKET?_

1. To allow direct passthrough of the consumer socket on Android through
   `adb forward localabstract` and allow host tools to directly talk to the
   on-device tracing service. Today both the Perfetto UI and Android GPU
   Inspector do this.
2. To allow in future to directly control a remote service over TCP or SSH
   tunneling.
3. Because the socket buffer for `SOCK_DGRAM` is extremely limited and
   and `SOCK_SEQPACKET` is not supported on MacOS.

_Why not gRPC?_

The team evaluated gRPC in late 2017 as an alternative but ruled it out
due to: (i) binary size and memory footprint; (ii) the complexity and overhead
of running a full HTTP/2 stack over a UNIX socket; (iii) the lack of
fine-grained control on back-pressure.

_Is the UNIX socket protocol used within Chrome processes?_

No. Within Chrome processes (the browser app, not CrOS) Perfetto doesn't use
any doesn't use any unix socket. Instead it uses the functionally equivalent
Mojo endpoints [`Producer{Client,Host}` and `Consumer{Client,Host}`][mojom].

### {#shmem-abi} Shared memory

This section describes the binary interface of the memory buffer shared between
a producer process and the tracing service (SMB).

The SMB is a staging area to decouple data sources living in the Producer
and allow them to do non-blocking async writes. A SMB is small-ish, typically
hundreds of KB. Its size is configurable by the producer when connecting.
For more architectural details about the SMB see also the
[buffers and dataflow doc](/docs/concepts/buffers.md) and the
[shared_memory_abi.h] sources.

#### Obtaining the SMB

The SMB is obtained by passing a tmpfs file descriptor over the producer socket
and memory-mapping it both from the producer and service.
The producer specifies the desired SMB size and memory layout when sending the
[`InitializeConnectionRequest`][producer_port.proto] request to the
service, which is the very first IPC sent after connection.
By default, the service creates the SMB and passes back its file descriptor to
the producer with the [`InitializeConnectionResponse`][producer_port.proto]
IPC reply. Recent versions of the service (Android R / 11) allow the FD to be
created by the producer and passed down to the service in the request. When the
service supports this, it acks the request setting
`InitializeConnectionResponse.using_shmem_provided_by_producer = true`. At the
time of writing this feature is used only by Chrome for dealing with lazy
Mojo initialization during startup tracing.

#### SMB memory layout: pages, chunks, fragments and packets

The SMB is partitioned into fixed-size pages. A SMB page must be an integer
multiple of 4KB. The only valid sizes are: 4KB, 8KB, 16KB, 32KB.

The size of a SMB page is determined by each Producer at connection time, via
the `shared_memory_page_size_hint_bytes` field of `InitializeConnectionRequest`
and cannot be changed afterwards. All pages in the SMB have the same size,
constant throughout the lifetime of the producer process.

![Shared Memory ABI Overview](/docs/images/shmem-abi-overview.png)

**A page** is a fixed-sized partition of the shared memory buffer and is just a
container of chunks.
The Producer can partition each Page SMB using a limited number of predetermined
layouts (1 page : 1 chunk; 1 page : 2 chunks and so on).
The page layout is stored in a 32-bit atomic word in the page header. The same
32-bit word contains also the state of each chunk (2 bits per chunk).

Having fixed the total SMB size (hence the total memory overhead), the page
size is a triangular trade off between:

1. IPC traffic: smaller pages -> more IPCs.
2. Producer lock freedom: larger pages -> larger chunks -> data sources can
   write more data without needing to swap chunks and synchronize.
3. Risk of write-starving the SMB: larger pages -> higher chance that the
   Service won't manage to drain them and the SMB remains full.

The page size, on the other side, has no implications on memory wasted due to
fragmentation (see Chunk below).

**A chunk** A chunk is a portion of a Page and contains a linear sequence of
[`TracePacket(s)`][trace-packet-ref] (the root trace proto).

A Chunk defines the granularity of the interaction between the Producer and
tracing Service. When a producer fills a chunk it sends `CommitData` IPC to the
service, asking it to copy its contents into the central non-shared buffers.

A a chunk can be in one of the following four states:

* `Free` : The Chunk is free. The Service shall never touch it, the Producer
   can acquire it when writing and transition it into the `BeingWritten` state.

* `BeingWritten`: The Chunk is being written by the Producer and is not
    complete yet (i.e. there is still room to write other trace packets).
    The Service never alter the state of chunks in the `BeingWritten` state
    (but will still read them when flushing even if incomplete).

* `Complete`: The Producer is done writing the chunk and won't touch it
  again. The Service can move it to its non-shared ring buffer and mark the
  chunk as `BeingRead` -> `Free` when done.

* `BeingRead`: The Service is moving the page into its non-shared ring
  buffer. Producers never touch chunks in this state.
  _Note: this state ended up being never used as the service directly
   transitions chunks from `Complete` back to `Free`_.

A chunk is owned exclusively by one thread of one data source of the producer.

Chunks are essentially single-writer single-thread lock-free arenas. Locking
happens only when a Chunk is full and a new one needs to be acquired.

Locking happens only within the scope of a Producer process.
Inter-process locking is not generally allowed. The Producer cannot lock the
Service and vice versa. In the worst case, any of the two can starve the SMB, by
marking all chunks as either being read or written. But that has the only side
effect of losing the trace data.
The only case when stalling on the writer-side (the Producer) can occur is when
a data source in a producer opts in into using the
[`BufferExhaustedPolicy.kStall`](/docs/concepts/buffers.md) policy and the SMB
is full.

**[TracePacket][trace-packet-ref]** is the atom of tracing. Putting aside
pages and chunks a trace is conceptually just a concatenation of TracePacket(s).
A TracePacket can be big (up to 64 MB) and can span across several chunks, hence
across several pages.
A TracePacket can therefore be >> chunk size, >> page size and even >> SMB size.
The Chunk header carries metadata to deal with the TracePacket splitting.

Overview of the Page, Chunk, Fragment and Packet concepts:<br>
![Shared Memory ABI concepts](/docs/images/shmem-abi-concepts.png)

Memory layout of a Page:<br>
![SMB Page layout](/docs/images/shmem-abi-page.png)

Because a packet can be larger than a page, the first and the last packets in
a chunk can be fragments.

![TracePacket spanning across SMB chunks](/docs/images/shmem-abi-spans.png)

#### Post-facto patching through IPC

If a TracePacket is particularly large, it is very likely that the chunk that
contains its initial fragments is committed into the central buffers and removed
from the SMB by the time the last fragments of the same packets is written.

Nested messages in protobuf are prefixed by their length. In a zero-copy
direct-serialization scenario like tracing, the length is known only when the
last field of a submessage is written and cannot be known upfront.

Because of this, it is possible that when the last fragment of a packet is
written, the writer needs to backfill the size prefix in an earlier fragment,
which now might have disappeared from the SMB.

In order to do this, the tracing protocol allows to patch the contents of a
chunk through the `CommitData` IPC (see
[`CommitDataRequest.ChunkToPatch`][commit_data_request.proto]) after the tracing
service copied it into the central buffer. There is no guarantee that the
fragment will be still there (e.g., it can be over-written in ring-buffer mode).
The service will patch the chunk only if it's still in the buffer and only if
the producer ID that wrote it matches the Producer ID of the patch request over
IPC (the Producer ID is not spoofable and is tied to the IPC socket file
descriptor).

### {#protos} Proto definitions

The following protobuf messages are part of the overall trace protocol ABI and
are updated maintaining backward-compatibility, unless marked as experimental
in the comments.

TIP: See also the _Updating A Message Type_ section of the
    [Protobuf Language Guide][proto-updating] for valid ABI-compatible changes
    when updating the schema of a protobuf message.

#### DataSourceDescriptor

Defined in [data_source_descriptor.proto]. This message is sent
Producer -> Service through IPC on the Producer socket during the Producer
initialization, before any tracing session is started. This message is used
to register advertise a data source and its capabilities (e.g., which GPU HW
counters are supported, their possible sampling rates).

#### DataSourceConfig

Defined in [data_source_config.proto]. This message is sent:

* Consumer -> Service through IPC on the Consumer socket, as part of the
  [TraceConfig](/docs/concepts/config.md) when a Consumer starts a new tracing
  session.

* Service -> Producer through IPC on the Producer socket, as a reaction to the
  above. The service passes through each `DataSourceConfig` section defined in
  the `TraceConfig` to the corresponding Producer(s) that advertise that data
  source.

#### TracePacket

Defined in [trace_packet.proto]. This is the root object written by any data
source into the SMB when producing any form of trace event.
See the [TracePacket reference][trace-packet-ref] for the full details.

## {#abi-stability} ABI Stability

All the layers of the tracing protocol ABI are long-term stable and can only
be changed maintaining backwards compatibility.

This is due to the fact that on every Android release the `traced` service
gets frozen in the system image while unbundled apps (e.g. Chrome) and host
tools (e.g. Perfetto UI) can be updated at a more frequently cadence.

Both the following scenarios are possible:

#### Producer/Consumer client older than tracing service

This happens typically during Android development. At some point some newer code
is dropped in the Android platform and shipped to users, while client software
and host tools will lag behind (or simply the user has not updated their app /
tools).

The tracing service needs to support clients talking and older version of the
Producer or Consumer tracing protocol.

* Don't remove IPC methods from the service.
* Assume that fields added later to existing methods might be absent.
* For newer Producer/Consumer behaviors, advertise those behaviors through
  feature flags when connecting to the service. Good examples of this are the
  `will_notify_on_stop` or `handles_incremental_state_clear` flags in
  [data_source_descriptor.proto]

#### Producer/Consumer client newer than tracing service

This is the most likely scenario. At some point in 2022 a large number of phones
will still run Android P or Q, hence running a snapshot of the tracing service
from ~2018-2020, but will run a recent version Google Chrome.
Chrome, when configured in system-tracing mode (i.e. system-wide + in-app
tracing), connects to the Android's `traced` producer socket and talks the
latest version of the tracing protocol.

The producer/consumer client code needs to be able to talk with an older version of the
service, which might not support some newer features.

* Newer IPC methods defined in [producer_port.proto] won't exist in the older
  service. When connecting on the socket the service lists its RPC methods
  and the client is able to detect if a method is available or not.
  At the C++ IPC layer, invoking a method that doesn't exist on the service
  causes the `Deferred<>` promise to be rejected.

* Newer fields in existing IPC methods will just be ignored by the older version
  of the service.

* If the producer/consumer client depends on a new behavior of the service, and
  that behavior cannot be inferred by the presence of a method, a new feature
  flag  must be exposed through the `QueryCapabilities` method.

## Static linking vs shared library

The Perfetto C++ Client Library is only available in the form of a static
library and a single-source amalgamated SDK (which is effectively a static
library). The library implements the Tracing Protocol ABI so, once statically
linked, depends only on the socket and shared memory protocol ABI, which are
guaranteed to be stable.

No shared library distributions for the C++ are available. We strongly
discourage teams from attempting to build the C++ tracing library as shared
library and use it from a different linker unit. It is fine to link AND use the
client library within the same shared library, as long as none of the perfetto
C++ API is exported.

The `PERFETTO_EXPORT_COMPONENT` annotations are only used when building the
third tier of the client library in chromium component builds and cannot be
easily repurposed for delineating shared library boundaries for the other two
API tiers.

This is because the C++ the first two tiers of the Client Library C++ API make
extensive use of inline headers and C++ templates, in order to allow the
compiler to see through most of the layers of abstraction.

Maintaining the C++ ABI across hundreds of inlined functions and a shared
library is prohibitively expensive and too prone to break in extremely subtle
ways. For this reason the team has ruled out shared library distributions for
the time being.

A new C Client library API/ABI is in the works, but it's not stable yet.

[cli_lib]: /docs/instrumentation/tracing-sdk.md
[selinux_producer]: https://cs.android.com/search?q=perfetto_producer%20f:sepolicy.*%5C.te&sq=
[selinux_consumer]:https://cs.android.com/search?q=f:sepolicy%2F.*%5C.te%20traced_consumer&sq=
[mojom]: https://source.chromium.org/chromium/chromium/src/+/master:services/tracing/public/mojom/perfetto_service.mojom?q=producer%20f:%5C.mojom$%20perfetto&ss=chromium&originalUrl=https:%2F%2Fcs.chromium.org%2F
[proto_rpc]: https://developers.google.com/protocol-buffers/docs/proto#services
[producer_port.proto]: /protos/perfetto/ipc/producer_port.proto
[consumer_port.proto]: /protos/perfetto/ipc/consumer_port.proto
[trace_packet.proto]: /protos/perfetto/trace/trace_packet.proto
[data_source_descriptor.proto]: /protos/perfetto/common/data_source_descriptor.proto
[data_source_config.proto]: /protos/perfetto/config/data_source_config.proto
[trace-packet-ref]: /docs/reference/trace-packet-proto.autogen
[shared_memory_abi.h]: /include/perfetto/ext/tracing/core/shared_memory_abi.h
[commit_data_request.proto]: /protos/perfetto/common/commit_data_request.proto
[proto-updating]: https://developers.google.com/protocol-buffers/docs/proto#updating