> Part of the walkerOS documentation. Project overview and full index: # Cache Cache is a built-in config property available on every step type (source, transformer, destination). It prevents redundant processing by caching results and serving them on subsequent matching requests. ## Configuration[​](#configuration "Direct link to Configuration") Add `cache` to any step in your flow config: ``` "sources": { "express": { "package": "@walkeros/server-source-express", "cache": { "rules": [ { "match": { "key": "ingest.method", "operator": "eq", "value": "GET" }, "key": ["ingest.method", "ingest.path"], "ttl": 300 } ] } } } ``` ### Cache Rule Properties[​](#cache-rule-properties "Direct link to Cache Rule Properties") | Property | Type | Description | | -------- | ----------------------- | -------------------------------------- | | `match` | `MatchExpression` | When to apply this cache rule | | `key` | `string[]` | Dot-paths to build the cache key from | | `ttl` | `number` | Time-to-live in seconds | | `update` | `Record` | Optional modifications applied on read | `match` is optional, omit it for an always-match rule. ### Cache Properties[​](#cache-properties "Direct link to Cache Properties") | Property | Type | Default | Description | | ----------- | ------------- | ---------------------------- | ------------------------------------------------------------------------------------------------------------------------- | | `stop` | `boolean` | `false` (`true` for sources) | Stop flow on cache HIT and skip remaining steps, returning the cached value | | `store` | `string` | Built-in memory | Reference to a store in `flow.stores` | | `namespace` | `string` | *(none)* | Optional key prefix. Omit to write keys directly to the store. Same store + same key + same namespace = same cache entry. | | `rules` | `CacheRule[]` | Required | At least one cache rule | ## Behavior by Step Type[​](#behavior-by-step-type "Direct link to Behavior by Step Type") ### Source Cache (stop by default)[​](#source-cache-stop-by-default "Direct link to Source Cache (stop by default)") Source cache intercepts requests before the pipeline runs. Sources default to `stop: true`. On cache HIT, the cached respond value is returned immediately and the pipeline is skipped. Set `stop: false` to skip only the source step and let the pipeline continue. ``` "sources": { "express": { "cache": { "rules": [ { "match": { "key": "ingest.method", "operator": "eq", "value": "GET" }, "key": ["ingest.method", "ingest.path"], "ttl": 300 } ] } } } ``` ### Transformer Cache (step-level)[​](#transformer-cache-step-level "Direct link to Transformer Cache (step-level)") Transformer cache skips the transformer's execution on HIT but continues the chain with the cached event. Useful for expensive computations like API enrichment. With `stop: true`, the remaining chain stops on HIT and downstream transformers and destinations are skipped. Halt scopes Pre-collector transformer caches with `stop: true` halt the entire pipeline on HIT, destinations do not see the event. Destination caches with `stop: true` halt only the per-destination delivery, sibling destinations still receive the event. ``` "transformers": { "enricher": { "package": "@walkeros/server-transformer-enricher", "cache": { "rules": [ { "key": ["event.name"], "ttl": 60 } ] } } } ``` ### Destination Cache (deduplication)[​](#destination-cache-deduplication "Direct link to Destination Cache (deduplication)") Destination cache skips the push on HIT, preventing duplicate events from reaching the destination. Other destinations still receive the event. With `stop: true`, the `before` transformer chain is also skipped on HIT. ``` "destinations": { "ga4": { "package": "@walkeros/web-destination-ga4", "cache": { "rules": [ { "key": ["event.name", "event.data.id"], "ttl": 300 } ] } } } ``` ## Key Resolution[​](#key-resolution "Direct link to Key Resolution") Cache keys use dot-path resolution from a structured context: * `ingest.method`: request method from source ingest * `ingest.path`: request path from source ingest * `event.name`: event name * `event.data.id`: nested event data field * `event.user.loggedIn`: user properties ## Update Rules[​](#update-rules "Direct link to Update Rules") Use `update` to modify the result on both HIT and MISS: ``` "cache": { "rules": [{ "key": ["ingest.method", "ingest.path"], "ttl": 300, "update": { "headers.X-Cache": { "key": "cache.status" }, "headers.Cache-Control": { "value": "public, max-age=300" } } }] } ``` `cache.status` resolves to `"HIT"` or `"MISS"`. Static values use `{ "value": "..." }`. ## Custom Store[​](#custom-store "Direct link to Custom Store") By default, cache uses a shared in-memory store. Reference a custom store for persistence: ``` { "stores": { "redis": { "package": "@walkeros/server-store-redis" } }, "sources": { "express": { "cache": { "store": "redis", "rules": [...] } } } } ``` Custom stores can be either synchronous (in-memory) or asynchronous (file system, Redis, HTTP, the cache wrapper). Cache lookups await the underlying `store.get` internally, so HIT/MISS semantics behave identically regardless of which backing you wire. ## Migration from prior v4 cache[​](#migration-from-prior-v4-cache "Direct link to Migration from prior v4 cache") If you're upgrading from an earlier v4 build, three breaking changes apply: * **`cache.full` is now `cache.stop`.** Search-and-replace `"full"` for `"stop"` in every `cache` block. Default behavior unchanged (sources default to `stop: true`, transformers and destinations default to `stop: false`). * **`cache.match: "*"` is now omitted.** Drop the `"match": "*"` entry from rules. A rule with no `match` matches every event, the behavior is identical to the previous wildcard. * **Implicit per-step namespacing is gone.** Previous builds prefixed cache keys with `s:`, `t:`, or `d:` based on step type, so a source and a destination writing the same key never collided. That prefix is no longer applied. If you share a store across step types and need isolation, set `cache.namespace` explicitly (for example `"namespace": "source"`).