---
title: The *processing component*
description: >
  Processing components are the units of incremental execution and the sync
  boundaries for target states. Covers mounting APIs (mount, use_mount,
  mount_each, mount_target), component paths, and granularity tradeoffs.
---
import MountVsUseMount from '/src/components/diagrams/concepts/MountVsUseMount.astro';

:::note[Prerequisite]
This page builds on [Core Concepts](./core_concepts), which introduces processing components, target states, and incremental sync **with diagrams** — the per-file fan-out and the chunk-embedding pipeline it walks through are referenced throughout this page. If the mounting APIs below feel abstract, start there.
:::

Most apps process many independent source items — files, rows, or entities. A **Processing Component** is the unit of execution for one: it runs that item's transformation logic and declares the set of **target states** produced for it.

## Component path

A **component path** is the stable identifier for a processing component across runs (think of it like a path in a tree). CocoIndex uses it to match a component to its previous run, detect what changed for that item, and sync that component's target states as a unit when it finishes. This sync happens per component; CocoIndex does not wait for other components in the same app to complete.

Component paths are hierarchical and form a tree structure. You specify child paths using `coco.component_subpath()` with stable identifiers like string literals, file names, row keys, or entity IDs:

```python
coco.component_subpath(filename)           # e.g., coco.component_subpath("hello.pdf")
coco.component_subpath("user", user_id)    # e.g., coco.component_subpath("user", 12345)
```

Choose paths that are stable for the "same" item (e.g., file path, primary key). If an item disappears and its path is no longer present, CocoIndex cleans up the target states owned by that path (and its sub-paths).

Here's an example component path tree for a pipeline that processes files:

```text
(root)                         ← app_main component
└── process_file
    ├── "hello.pdf"            ← process_file component
    └── "world.pdf"            ← process_file component
```

The tree is populated dynamically as the app runs — each `mount()` / `mount_each()` call adds a subpath.

See [StableKey](./sdk_overview#stablekey) in the SDK Overview for details on what values can be used in component paths.

## Mount

Mounting is how you declare (instantiate) a processing component within an app at a specific path, so CocoIndex knows that component exists, should run, and owns a set of target states — and can match it against its previous run to sync only what changed.

CocoIndex provides two core mounting APIs:

- **`mount()`** — sets up a processing component in a child path without depending on data from it. This allows the component to refresh independently in live mode.
- **`use_mount()`** — returns a value from the component's execution to the caller. The component at that path cannot refresh independently without re-executing the caller.

<MountVsUseMount />

**Which one you reach for comes down to a single question: does the caller need a value back?** `use_mount()` consumes the child's return value, which *couples* the two — the call blocks until the child finishes and commits its target states, and the child can't refresh on its own without re-running the caller. `mount()` takes nothing back and returns as soon as the child is scheduled, so the child runs on its own and can [refresh independently in live mode](./live_mode) when its inputs change.

And two sugar APIs that simplify common patterns:

- **`mount_each()`** — mounts one component per item in a keyed iterable
- **`mount_target()`** — mounts a target without an explicit subpath

See also [`map()`](#map) for a utility API that operates within a component without creating new ones.

### Automatic subpath derivation
`mount()`, `use_mount()`, and `mount_each()` all accept an optional `ComponentSubpath` as their first argument. When omitted, the subpath is **auto-derived** from the function name using `Symbol(fn.__name__)`.

```python
# These are equivalent:
await coco.mount(process_file, file, target)
await coco.mount(coco.component_subpath(coco.Symbol("process_file")), process_file, file, target)
```

This means the component path for a `process_file` function is `parent / Symbol("process_file")`. The function must have a `__name__` attribute; if it doesn't (e.g., a lambda), provide an explicit subpath.

Since sibling component paths must not collide, you need an explicit subpath when:
- **The same function is mounted more than once** — auto-derived paths would be identical, so each call needs a distinct path (e.g., `coco.component_subpath("session", youtube_id)`)
- **Different functions happen to share a `__name__`** — rare, but possible with wrappers or closures
- **You want a specific path name** — different from the function name

### `mount()`

Use `mount()` when you don't need a return value from the processing component. It schedules the processing component to run and returns a handle:

```python
handle = await coco.mount(process_file, file, target)
```

With an explicit subpath, for example when mounting multiple components of the same function:

```python
handle = await coco.mount(
    coco.component_subpath("process", filename),
    process_file,
    file,
    target,
)
```

The handle provides a method you can call if you need to wait until the processing component is fully ***ready*** — meaning all its target states have been applied to external systems and all components in its sub-paths are ready:

```python
await handle.ready()
```

You usually only need to call `ready()` when you have logic that depends on the processing component's target states being applied — for example, querying the latest data from a target table after syncing it.

`mount()` also accepts **LiveComponent classes** — components that process continuously and react to changes incrementally instead of rescanning everything. See [Live Components](../advanced_topics/live_component) for details.

### `use_mount()`

Use `use_mount()` when you need the processing component's return value. It mounts the component, waits until it's ready, and returns the value directly:

```python
table = await coco.use_mount(setup_table, table_name="docs")
```

With an explicit subpath:

```python
table = await coco.use_mount(
    coco.component_subpath("setup"),
    setup_table,
    table_name="docs",
)
```

A common use of `use_mount()` is to obtain a [target](./target_state#where-do-targets-come-from) after its container target state is applied.

### `mount_each()`
`mount_each()` mounts one processing component per item in a keyed iterable.

```python
files = localfs.walk_dir(sourcedir, path_matcher=PatternFilePathMatcher(included_patterns=["**/*.md"]))
await coco.mount_each(process_file, files.items(), target)
```

Each item in the iterable is a `(key, value)` tuple. The value is passed as the first argument to the function, and any additional arguments are passed through. Items are mounted under an [auto-derived subpath](#automatic-subpath-derivation) (`Symbol(fn.__name__)`), so the component path for each item is `parent / Symbol("process_file") / key`.

In the static case this is just the keyed-loop form of [`mount()`](#mount) — the same per-file fan-out shown in [Core Concepts](./core_concepts#processing-component). The snippet above is equivalent to looping over the items and calling `mount(coco.component_subpath(coco.Symbol("process_file"), key), process_file, value, target)` for each one. The per-item `key` is what keeps each component's path stable across runs, so the engine matches, updates, and cleans up each item individually.

You can provide an explicit subpath as the first argument:

```python
await coco.mount_each(coco.component_subpath("files"), process_file, files.items(), target)
```

Source connectors provide an `items()` method that returns `(StableKey, T)` pairs. For example, `localfs.walk_dir(...).items()` yields `(relative_path, File)` tuples.

It also does one thing a hand-written loop can't: when a source connector supports live watching, its `items()` returns a `LiveMapView` or `LiveMapFeed` instead of a plain iterable, and `mount_each()` detects this and automatically adds, updates, and removes per-item components as the source changes — no changes to `mount_each()` itself are needed. See [Live Mode](./live_mode).

### `mount_target()`
`mount_target()` mounts a target without requiring an explicit subpath.

```python
from cocoindex.connectors import localfs

dir_target = await coco.mount_target(localfs.dir_target(outdir))
```

The component path is derived automatically from the target's globally unique key — you don't need to create a `component_subpath` for it. This is sugar over calling `use_mount()` with a target declaration function.

Connectors also provide convenience methods that wrap `mount_target()`:

```python
# Equivalent to the above
dir_target = await localfs.mount_dir_target(outdir)

# PostgreSQL example
table = await postgres.mount_table_target(
    PG_DB,
    table_name="doc_embeddings",
    table_schema=await postgres.TableSchema.from_class(DocEmbedding, primary_key=["id"]),
)
```

### Using `component_subpath` as a context manager

You can use `component_subpath()` as a context manager to create nested paths without repeating common prefixes:

```python
with coco.component_subpath("process"):
    for f in files:
        await coco.mount(
            coco.component_subpath(str(f.relative_path)),
            process_file,
            f,
            target,
        )
```

This is equivalent to:

```python
for f in files:
    await coco.mount(
        coco.component_subpath("process", str(f.relative_path)),
        process_file,
        f,
        target,
    )
```

:::tip
When iterating over keyed items, prefer [`mount_each()`](#mount_each) — it handles the loop and subpath creation for you.
:::

## How target states sync

The component path tree determines ownership. When a component is no longer mounted at a path (e.g., a source file is deleted), CocoIndex automatically cleans up its target states — and recursively for all its sub-paths.

:::info[Sync Mechanism]

After a processing component finishes, CocoIndex syncs its target states:

1. **Compares** the target states declared in this run against those from the previous run at the same path
2. **Applies changes** as a unit — creating, updating, or deleting target states as needed
3. **Recursively cleans up** sub-paths where components are no longer mounted

All writes happen strictly after processing completes — you never see partial effects from a processing failure or interrupt. Each target backend applies its batch atomically when supported (e.g., within a database transaction), but changes across different target backends are not transactional with each other.

:::

## What happens when a component fails

CocoIndex processes each component in two phases: **processing** (running your function, declaring target states) and **submit** (writing changes to target backends).

### Failure isolation

The framework's general rule: **a call raises on failure iff the failed work was on the critical path for the call to return.**

- **`use_mount()`** — you're awaiting the child's *value*, so the child succeeding is on the critical path. The child's exception propagates to the parent.
- **`mount()` and `mount_each()`** — these return as soon as the work is *scheduled* (you get a handle back). The child's execution runs in the background, off your critical path. A failure in one child does **not** affect the parent or siblings — by default the exception is logged and other components continue. One bad file shouldn't take down the entire pipeline.

To react to background failures, you can:

- Install [exception handlers](../advanced_topics/exception_handlers#exception-handlers) — global or scoped — to send alerts, record metrics, or implement custom logic. A handler that raises propagates the failure through `await handle.ready()` if you choose to await it.
- [Monitor app progress](../advanced_topics/progress_monitoring) — `UpdateStats` exposes per-component stats including error counts, so you can detect failures programmatically.

For the full picture — including the critical-path principle applied to every API, interrupted update recovery, and the exception handler API — see [Error Handling](../advanced_topics/exception_handlers).

### No rollback, convergent roll-forward

CocoIndex does not roll back partial writes. The two-phase design makes this safe:

- **Processing** is side-effect-free — it only declares target states in memory. If processing fails (e.g., a parsing error), no writes were attempted, so there's nothing to undo.
- **Submit** writes changes to target backends. If a submit fails partway through (e.g., a database connection drops), some writes may have been applied. CocoIndex does not attempt to undo them. Instead, on the next run CocoIndex computes the current desired state, and the target connector reconciles against all possible previous states — converging the target to the correct state regardless of what was partially applied. This is why built-in connectors use convergent operations like upserts (`INSERT ... ON CONFLICT DO UPDATE`) rather than plain inserts.

## How big should a processing component be?

When defining processing components, think about granularity — what one path represents — because it determines the sync boundary for target states.

For example, if you're processing files:

- **Coarse**: one component for "all files" (`coco.component_subpath("process")`)
- **Medium**: one component per file (`coco.component_subpath("process", file_path)`)
- **Fine**: one component per chunk (`coco.component_subpath("process", file_path, chunk_id)`)

In general:

- **Coarse-grained** (fewer, larger components): More target states sync together as a unit, but you only see updates after the larger component finishes.
- **Fine-grained** (more, smaller components): Each component syncs its target states as soon as it finishes, but target states owned by different components do not sync together as a unit.

For small datasets, a single processing component that owns all target states is simple and ensures all target states sync together. As data grows, consider breaking it down into one component per source item (e.g., one per file) to reduce latency: you see each item's target states synced as soon as it's processed, without waiting for the full dataset to complete. This also helps isolate failures to that item.

## Explicit context management

CocoIndex automatically propagates component context through Python's `contextvars`, which works for ordinary function calls (both sync and async). However, in situations where context variables are not preserved (for example, when using `concurrent.futures.ThreadPoolExecutor`), you need to explicitly capture and attach the context.

Use `coco.get_component_context()` to capture the current context, and `context.attach()` to restore it:

```python
from concurrent.futures import ThreadPoolExecutor

@coco.fn
def app_main() -> None:
    # Capture the current context
    ctx = coco.get_component_context()

    def worker(item):
        # Attach the context in the worker thread
        with ctx.attach():
            # Now CocoIndex APIs work correctly
            process_item(item)

    with ThreadPoolExecutor() as executor:
        executor.map(worker, items)
```

This pattern ensures that CocoIndex can track component relationships and target state ownership even across thread boundaries.

## Processing helpers

### `map()`
`map()` applies an async function to each item in a collection, running all calls concurrently within the current processing component. Unlike [`mount()`](#mount) and [`mount_each()`](#mount_each), it does **not** create child processing components — it's purely concurrent execution (similar to `asyncio.gather()`).

```python
@coco.fn(memo=True)
async def process_file(file: FileLike, table: postgres.TableTarget[DocEmbedding]) -> None:
    chunks = splitter.split(await file.read_text())
    id_gen = IdGenerator()
    await coco.map(process_chunk, chunks, file.file_path.path, id_gen, table)
```

The first argument to the function receives each item; additional arguments are passed through to every call. `map()` returns a `list` of the results, in the same order as the input items.

#### When to use `map()` vs `mount_each()`

- Use **`mount_each()`** when each item should be its own processing component — with its own component path, target state ownership, and target states sync boundary.
- Use **`map()`** when you want to process items concurrently *within* the current component, without creating new component boundaries. This is common for sub-item work like processing chunks within a file — the same within-component chunk work the [chunk-embedding pipeline in Core Concepts](./core_concepts#processing-component) walks through.
