# General Information
Processors are used at various stages of a [pipeline](../docs/pipelines.md).
Generally, a processor pre-processes data before it is exported (e.g.
modify attributes or sample) or helps ensure that data makes it through a
pipeline successfully (e.g. batch/retry).

Some important aspects of pipelines and processors to be aware of:
- [Data Ownership](#data-ownership)
- [Exclusive Ownership](#exclusive-ownership)
- [Shared Ownership](#shared-ownership)
- [Ordering Processors](#ordering-processors)
- [Recommended Processors](#recommended-processors)

Supported processors (sorted alphabetically):
- [Attributes Processor](attributesprocessor/README.md)
- [Batch Processor](batchprocessor/README.md)
- [Filter Processor](filterprocessor/README.md)
- [Memory Limiter Processor](memorylimiter/README.md)
- [Queued Retry Processor](queuedprocessor/README.md)
- [Resource Processor](resourceprocessor/README.md)
- Sampling Processors
  - [Probabilistic Sampling Processor](samplingprocessor/probabilisticsamplerprocessor/README.md)
  - [Tail Sampling Processor](samplingprocessor/tailsamplingprocessor/README.md)
- [Span Processor](spanprocessor/README.md)

The [contributors repository](https://github.com/open-telemetry/opentelemetry-collector-contrib)
 has more processors that can be added to custom builds of the Collector.

## <a name="data-ownership"></a>Data Ownership

The ownership of the `TraceData` and `MetricsData` in a pipeline is passed as the data travels
through the pipeline. The data is created by the receiver and then the ownership is passed
to the first processor when `ConsumeTraceData`/`ConsumeMetricsData` function is called.

Note: the receiver may be attached to multiple pipelines, in which case the same data
will be passed to all attached pipelines via a data fan-out connector.

From data ownership perspective pipelines can work in 2 modes:
* Exclusive data ownership
* Shared data ownership

The mode is defined during startup based on data modification intent reported by the
processors. The intent is reported by each processor via `MutatesConsumedData` field of
the struct returned by `GetCapabilities` function. If any processor in the pipeline
declares an intent to modify the data then that pipeline will work in exclusive ownership
mode. In addition, any other pipeline that receives data from a receiver that is attached
to a pipeline with exclusive ownership mode will be also operating in exclusive ownership
mode.

### <a name="exclusive-ownership"></a>Exclusive Ownership

In exclusive ownership mode the data is owned exclusively by a particular processor at a
given moment of time and the processor is free to modify the data it owns.

Exclusive ownership mode is only applicable for pipelines that receive data from the
same receiver. If a pipeline is marked to be in exclusive ownership mode then any data
received from a shared receiver will be cloned at the fan-out connector before passing
further to each pipeline. This ensures that each pipeline has its own exclusive copy of
data and the data can be safely modified in the pipeline.

The exclusive ownership of data allows processors to freely modify the data while
they own it (e.g. see `attributesprocessor`). The duration of ownership of the data
by processor is from the beginning of `ConsumeTraceData`/`ConsumeMetricsData` call
until the processor calls the next processor's `ConsumeTraceData`/`ConsumeMetricsData`
function, which passes the ownership to the next processor. After that the processor
must no longer read or write the data since it may be concurrently modified by the
new owner.

Exclusive Ownership mode allows to easily implement processors that need to modify
the data by simply declaring such intent.

### <a name="shared-ownership"></a>Shared Ownership

In shared ownership mode no particular processor owns the data and no processor is
allowed the modify the shared data.

In this mode no cloning is performed at the fan-out connector of receivers that
are attached to multiple pipelines. In this case all such pipelines will see
the same single shared copy of the data. Processors in pipelines operating in shared
ownership mode are prohibited from modifying the original data that they receive
via `ConsumeTraceData`/`ConsumeMetricsData` call. Processors may only read the data but
must not modify the data.

If the processor needs to modify the data while performing the processing but
does not want to incur the cost of data cloning that Exclusive mode brings then
the processor can declare that it does not modify the data and use any
different technique that ensures original data is not modified. For example,
the processor can implement copy-on-write approach for individual sub-parts of
`TraceData`/`MetricsData` argument. Any approach that does not mutate the
original `TraceData`/`MetricsData` argument (including referenced data, such as
`Node`, `Resource`, `Spans`, etc) is allowed.

If the processor uses such technique it should declare that it does not intend
to modify the original data by setting `MutatesConsumedData=false` in its capabilities
to avoid marking the pipeline for Exclusive ownership and to avoid the cost of
data cloning described in Exclusive Ownership section.

## <a name="ordering-processors"></a>Ordering Processors

The order processors are specified in a pipeline is important as this is the
order in which each processor is applied to traces and metrics.

### Include/Exclude Metrics

The [filter processor](filterprocessor/README.md) exposes the option to provide a set of
metric names to match against to determine if the metric should be
included or excluded from the processor. To configure this option, under
`include` and/or `exclude` both `match_type` and `metrics_names` are required.

Note: If both `include` and `exclude` are specified, the `include` properties
are checked before the `exclude` properties.

```yaml
filter:
    # include and/or exclude can be specified. However, the include properties
    # are always checked before the exclude properties.
    {include, exclude}:
      # match_type controls how items matching is done.
      # Possible values are "regexp" or "strict".
      # This is a required field.
      match_type: {strict, regexp}

      # regexp is an optional configuration section for match_type regexp.
      regexp:
        # < see "Match Configuration" below >

      # metric_names specify an array of items to match the metric name against.
      # This is a required field.
      metric_name: [<item1>, ..., <itemN>]
```

#### Match Configuration

Some `match_type` values have additional configuration options that can be
specified. The `match_type` value is the name of the configuration section.
These sections are optional.

```yaml
# regexp is an optional configuration section for match_type regexp.
regexp:
  # cacheenabled determines whether match results are LRU cached to make subsequent matches faster.
  # Cache size is unlimited unless cachemaxnumentries is also specified.
  cacheenabled: <bool>
  # cachemaxnumentries is the max number of entries of the LRU cache; ignored if cacheenabled is false.
  cachemaxnumentries: <int>
```

### Include/Exclude Spans

The [attribute processor](attributesprocessor/README.md) and the [span processor](spanprocessor/README.md) expose
the option to provide a set of properties of a span to match against to determine
if the span should be included or excluded from the processor. To configure
this option, under `include` and/or `exclude` at least `match_type` and one of
`services`, `span_names` or `attributes` is required.

Note: If both `include` and `exclude` are specified, the `include` properties
are checked before the `exclude` properties.

```yaml
{span, attributes}:
    # include and/or exclude can be specified. However, the include properties
    # are always checked before the exclude properties.
    {include, exclude}:
      # At least one of services, span_names or attributes must be specified.
      # It is supported to have more than one specified, but all of the specified
      # conditions must evaluate to true for a match to occur.

      # match_type controls how items in "services" and "span_names" arrays are
      # interpreted. Possible values are "regexp" or "strict".
      # This is a required field.
      match_type: {strict, regexp}

      # regexp is an optional configuration section for match_type regexp.
      regexp:
        # < see "Match Configuration" below >

      # services specify an array of items to match the service name against.
      # A match occurs if the span service name matches at least of the items.
      # This is an optional field.
      services: [<item1>, ..., <itemN>]

      # The span name must match at least one of the items.
      # This is an optional field.
      span_names: [<item1>, ..., <itemN>]

      # Attributes specifies the list of attributes to match against.
      # All of these attributes must match exactly for a match to occur.
      # Only match_type=strict is allowed if "attributes" are specified.
      # This is an optional field.
      attributes:
          # Key specifies the attribute to match against.
        - key: <key>
          # Value specifies the exact value to match against.
          # If not specified, a match occurs if the key is present in the attributes.
          value: {value}
```

#### Match Configuration

Some `match_type` values have additional configuration options that can be
specified. The `match_type` value is the name of the configuration section.
These sections are optional.

```yaml
# regexp is an optional configuration section for match_type regexp.
regexp:
  # cacheenabled determines whether match results are LRU cached to make subsequent matches faster.
  # Cache size is unlimited unless cachemaxnumentries is also specified.
  cacheenabled: <bool>
  # cachemaxnumentries is the max number of entries of the LRU cache; ignored if cacheenabled is false.
  cachemaxnumentries: <int>
```

## <a name="recommended-processors"></a>Recommended Processors

No processors are enabled by default, however multiple processors are recommended to
be enabled. These are:

1. memory_limiter
2. *any sampling processors*
3. batch
4. *any other processors*
5. queued_retry

In addition, it is important to note that the order of processors matters. The order
above is the best practice. Refer to the individual processor documentation below
for more information.
