Documentation Index
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Construct a whole-pipe transformer.
createTransformer<I, O>(config: TransformerOptions<I, O>): Transformer<I, O>
transform: (required) (data: I, ctx: BatchContext) => O | Promise<O>. Called once per batch.start: (optional) (ctx: StartCtx) => void | Promise<void>. Called once when the pipe starts. Use this to load state, warm up caches, or query the portal for historical data before the main stream begins.stop: (optional) (ctx: StopCtx) => void | Promise<void>. Called once when the pipe stops.fork: (optional) (cursor: BlockCursor, ctx: Ctx) => void | Promise<void>. Called before the next batch whenever the source detects a chain reorg. cursor identifies the last safe . See Fork handling.profiler: (optional) { name: string; hidden?: boolean }. Overrides the transformer’s node name in the profiler tree.
Example:
Context variables
Each callback receives a context object. The fields differ by callback.
transform(data, ctx: BatchContext)
ctx is the full per-batch context. Fields:
| Field | Type | Description |
|---|
id | string | Pipeline ID — the id passed to fuelPortalStream(). |
logger | Logger | Pino-compatible logger scoped to this batch. Defaults to the source-level logger. |
metrics | Metrics | Prometheus metrics registry. Use ctx.metrics.counter(), .gauge(), .histogram(), .summary() to register and update custom metrics. See Metrics. |
profiler | Profiler | Open a span with ctx.profiler.start('label'). See Profiling. |
stream | BatchStreamContext | Per-stream state (see below). |
batch | BatchMetadata | Per-batch volume info (see below). |
ctx.stream: BatchStreamContext
| Field | Type | Description |
|---|
dataset | ApiDataset | Dataset metadata returned by the portal (chain name, genesis, tier). |
head.finalized | BlockCursor | undefined | Current finalized head known to the portal, if advertised. |
head.latest | BlockCursor | undefined | Current unfinalized head. |
state.initial | number | First the stream was configured to read. |
state.last | number | Last the stream intends to read (often Infinity). |
state.current | BlockCursor | Latest in this batch. Cursor has { number, hash?, timestamp? }. |
state.rollbackChain | BlockCursor[] | Unfinalized-chain tail — cursors the stream will need to roll back if a fork is detected. |
progress | ProgressEvent['progress'] | Progress metrics when progress is configured on the source; otherwise undefined. |
query | { url, hash, raw } | Debug info for the portal query feeding this batch. |
| Field | Type | Description |
|---|
blocksCount | number | Number of in this batch. |
bytesSize | number | Compressed payload size received from the portal. |
requests | Record<number, number> | Map of HTTP status code → number of responses that produced this batch. |
lastBlockReceivedAt | Date | Wall-clock time the last block was received. |
start(ctx: StartCtx)
Fired once, before any batch. Use to warm up caches or run one-off queries.
| Field | Type | Description |
|---|
id | string | Pipeline ID. |
logger | Logger | Same as in BatchContext. |
metrics | Metrics | Same as in BatchContext. |
portal | PortalClient | Live portal client. Use portal.getStream(query) for warm-up reads. |
state.initial | number | First the stream was configured to read. |
state.current | BlockCursor | undefined | Cursor persisted by the previous run, if any. undefined on a fresh start. |
fork(cursor, ctx: Ctx)
Fired before the next batch whenever a reorg is detected. cursor is the last to keep; drop state produced for anything after it.
| Field | Type | Description |
|---|
logger | Logger | Same as in BatchContext. |
profiler | Profiler | Same as in BatchContext. |
stop(ctx: StopCtx)
Fired once when the pipe stops.
| Field | Type | Description |
|---|
logger | Logger | Same as in BatchContext. |
