source

Connectors (internal/source)

A connector is a data source adapter: it exposes one external system (the GitHub API, a Jaeger instance, …) as one or more SQL tables under a schema name. This package defines the connector contract; each connector lives in its own subpackage (internal/source/github, internal/source/jaeger).

This document is the reference for writing a new connector. It is detailed on purpose — enough that you can hand it to an agent and have it produce a sound implementation plan. Read it top to bottom before starting; the Step-by-step and Checklist sections at the end are the actionable summary.

Contents

• Where a connector fits
• The contract
• The query lifecycle in detail
• Tables and columns → SQLite
• Push-down
• Streaming with emit
• Registration and configuration
• Observability
• Testing
• Reference implementations
• Invariants and gotchas
• Step-by-step
• Checklist

Where a connector fits

dfetch resolves a query like this (see internal/engine/engine.go):

1. Parse the SQL into a typed AST (internal/sqlparse).
2. Resolve each schema-qualified table (github.issues) to the connector registered under that schema, and to that connector's TableSchema.
3. For each source: attach an on-disk SQLite database under the schema name and create the table from the connector's declared columns.
4. Scan every connector concurrently, loading each emitted chunk into its SQLite table as it arrives.
5. Run the original SQL verbatim against the loaded SQLite database and return the result.

The crucial consequence of step 5: SQLite is the source of truth. A connector only has to return a superset of the rows the query needs — the engine re-applies the full WHERE/JOIN/ORDER BY/LIMIT locally. Push-down is therefore a pure optimization (fetch less), never a correctness requirement.

The contract

Everything is in internal/source/source.go.

// Connector exposes the tables of one external system.
type Connector interface {
    // Tables returns the schema of every table this connector serves.
    Tables() []TableSchema

    // Scan fetches rows for one table (req.Table), pushing down what it can from
    // req. It calls emit once per chunk (e.g. per API page) so the engine can
    // load each chunk as it arrives instead of buffering the whole result.
    // Returning an error from emit aborts the scan; Scan should propagate it.
    Scan(ctx context.Context, req ScanRequest, emit func(*Rows) error) error
}

// Factory builds a Connector from its config params (nil for a builtin).
type Factory func(params map[string]any) (Connector, error)

Supporting types:

type Column struct {
    Name string // column name in the SQLite table
    Type string // SQLite affinity: "TEXT", "INTEGER", "REAL" (empty → TEXT)
}

type TableSchema struct {
    Name    string
    Columns []Column
}
func (t TableSchema) ColumnNames() []string // names in order

type ScanRequest struct {
    Table   string       // which table to scan (you serve one per Scan call)
    Columns []string     // projected columns; empty means all. Optional to honor.
    Filters []Filter     // WHERE conjuncts attributable to this source
    OrderBy []OrderTerm  // ORDER BY terms attributable to this source
    Limit   *int         // LIMIT, when safe to push (nil otherwise)
    Offset  *int         // OFFSET, when safe to push (nil otherwise)
}
func (r ScanRequest) Filter(column string) (Filter, bool) // first filter on column

type Filter struct {
    Column string
    Op     sqlparse.Operator // OpEq, OpGt, OpBetween, OpIn, … (see below)
    Value  any               // single-value ops: string/int64/float64/bool/[]byte/nil
    Values []any             // IN list, or [low, high] for BETWEEN
}

type OrderTerm struct {
    Column string
    Desc   bool
}

// Rows is one emitted chunk: column names plus rows whose values are ordered to
// match Columns.
type Rows struct {
    Columns []string
    Rows    [][]any
}

A connector typically also exposes a New(params map[string]any) (source.Connector, error) that matches Factory.

Dynamic sources (lazy schema)

Connectors with a small, fixed set of tables (the API connectors here) just return them all from Tables(). A dynamic source — a SQL warehouse like Postgres or Snowflake with hundreds of tables — should not enumerate its whole catalog to run one query or to power dfetch tables. Such a connector returns an empty (or curated) Tables() and implements two optional capabilities, which the engine detects by type assertion:

// Resolve one table's columns on demand. The engine prefers this over the
// Tables() lookup, so only the tables a query references are introspected.
// found == false (nil error) means "no such table".
type SchemaDescriber interface {
    DescribeTable(ctx context.Context, table string) (ts TableSchema, found bool, err error)
}

// List table names (no columns) for discovery, optionally filtered.
type TableLister interface {
    ListTables(ctx context.Context, opts ListOptions) ([]string, error)
}

type ListOptions struct {
    Filter string // case-insensitive substring on the table name; "" = all
    Limit  int    // cap on names returned; 0 = no limit
}

The engine still creates each referenced table up front, so DescribeTable must return the full column list for the table being resolved (it runs during resolution, before Scan). dfetch tables uses these for its tiered view: schemas + counts → ListTables names → DescribeTable columns.

Scoping via config. Bound what a dynamic source exposes through params (no special config schema — just documented keys your New reads). The conventional keys are schemas (namespaces to expose) and tables (an explicit allowlist); parse them with the source.StringList(params, key) helper, which tolerates a YAML list or a single string:

sources:
  - name: warehouse
    type: postgres
    params:
      schemas: [public, analytics]
      tables: [public.orders, public.users] # optional allowlist

The query lifecycle in detail

What the engine does around your connector (internal/engine/engine.go, plan.go):

• Resolution (resolveSource): the schema qualifier picks the connector; the table name is matched against conn.Tables(). An unqualified table, an unknown schema, or an unknown table is a user-facing error before any scan.
• Table creation happens up front and serially (attach + the attached-schema map are not goroutine-safe), via localdb.CreateTable.
• Scanning runs all sources concurrently in an errgroup (cap 8). Each Scan runs inside a connector.scan span. The emit callback writes the chunk with localdb.Insert, serialized by a mutex because localdb uses a single pinned connection. The first error cancels the rest.
• Planning (planScan) builds the ScanRequest for each source — see Push-down.

You implement Tables() and Scan(). You do not touch localdb, the engine, or SQLite directly.

Tables and columns → SQLite

localdb.CreateTable turns your TableSchema into CREATE TABLE "schema"."table" ("col" TYPE, …):

• Column.Type is used verbatim as the SQLite column type; empty defaults to TEXT. Use TEXT, INTEGER, or REAL (SQLite type affinities). No constraints, PKs, or NOT NULL are emitted.
• Pick column names that mirror the source's own field names so queries read like the upstream API/docs (created_at, user_login, duration_ms).

localdb.Insert loads each emitted chunk:

• Every row's value count must equal the table's column count, in the same order as the declared columns — a mismatch is an error. Build rows positionally.
• Accepted Go value types: string, int64, float64, bool (stored as 0/1), []byte, and nil (SQL NULL). Use nil for absent/null fields. Prefer int64 over int for integer columns.
• On read-back, []byte results are normalized to strings.

There is no schema migration: each request creates fresh tables, so changing your columns is free.

Push-down

planScan (internal/engine/plan.go) decides what to offer each source in the ScanRequest. You then decide what to actually honor.

What the planner offers:

• Filters whose column belongs to the table and is attributable to this source (qualified with the source's alias/name, or unqualified in a single-source query). Structured comparisons are converted to Filter; OpNone (unparsed), IS NULL, and IS NOT NULL are not offered. Bind parameters (?, :id) are never pushed. The planner also infers equality filters across equi-joins (e.g. r.name = p.repo + p.repo = 'go' ⇒ r.name = 'go').
• OrderBy terms attributable to the source.
• Limit/Offset only when this source drives the result (single-source query, or a join where the LIMIT can safely ride the driving source).

Operators (sqlparse.Operator, see internal/sqlparse/ast.go): OpEq, OpNotEq, OpLt, OpLte, OpGt, OpGte, OpLike/OpNotLike, OpGlob/OpNotGlob, OpRegexp/OpNotRegexp, OpMatch/OpNotMatch, OpIs/OpIsNot/OpIsDistinctFrom/OpIsNotDistinctFrom, OpBetween/OpNotBetween (Values = [low, high]), OpIn/OpNotIn (Values = list). For single-value ops read Filter.Value; for IN/BETWEEN read Filter.Values.

The golden rule: only translate a filter/order/limit into an upstream request parameter when doing so still returns exactly the rows the query wants, or a superset. If the API can't honor something precisely, ignore it and let SQLite finish the job. Concretely:

• Equality on a column the API filters on → safe to push.
• A LIMIT is only safe to push when the API returns exactly the filtered + ordered set — i.e. every filter was consumed by the API and the ordering was fully honored. A multi-key ORDER BY the API can't reproduce must not push LIMIT, or you'll truncate rows the real order would keep. With an OFFSET, fetch limit+offset rows so SQLite can apply the offset.
• A required API parameter (GitHub's owner/repo, Jaeger's service_name): if it's missing from the filters, return a helpful error rather than fetching the world. Use the requireStringEq-style pattern.

When push-down is unsafe, cap unbounded fetches some other way (e.g. a max-pages limit) so a broad query doesn't pull an entire dataset.

Streaming with emit

Scan should call emit once per natural chunk — typically one API page — rather than buffering everything. The engine inserts each chunk as it arrives, so streaming reduces peak memory and overlaps network with local inserts.

• Emit &source.Rows{Columns: <table column names>, Rows: <[][]any>}. Keep Columns consistent across chunks (the engine uses the first chunk's columns).
• If emit returns an error, stop and propagate it (the engine is cancelling — e.g. another source failed).
• Skip empty chunks (don't emit a Rows with zero rows) — the one exception is a warning-only chunk from source.Warn(...), which carries Warnings and no rows to tell the user the result may be incomplete (e.g. a pagination/result cap was hit). The engine collects its Warnings and skips the (no-op) insert.
• Thread ctx into every outbound call for cancellation and tracing.

Registration and configuration

Two independent ways a connector becomes available:

Builtin — available with no config, under a fixed schema name. Add it to the builtins map in internal/engine/engine.go (and import the package):

var builtins = map[string]source.Factory{
    "github": github.New,
    "jaeger": jaeger.New,
    "<name>": myconnector.New, // now queryable as <name>.<table>
}

Builtins are constructed with factory(nil), so New must work with nil params and sensible defaults.

Config — a user binds any registered connector type to a schema name in dfetch.yaml (./dfetch.yaml, falling back to ~/dfetch.yaml; see internal/config). This can add new schemas or override a builtin, and lets one connector type serve several hosts:

sources:
  - name: gh-enterprise # schema; queried as gh-enterprise.issues
    type: github # registered connector type
    params:
      base_url: https://github.example.com/api/v3

engine.New registers the builtins, then builds each config source via the registry and stores it under its name (config wins on conflicts).

Params and secrets: read structured options from params (type-assert, e.g. params["base_url"].(string)); read secrets/tokens from the environment, not config (e.g. os.Getenv("GITHUB_TOKEN")). Always accept a base_url-style override — it's what lets tests point at a local httptest server.

Observability

• Build your HTTP client with otelhttp.NewTransport(http.DefaultTransport) so each request becomes a client span (a no-op until a tracer provider is installed). See github.New.
• Thread the ctx from Scan through every request (http.NewRequestWithContext) so spans nest under the engine's connector.scan span and cancellation works.
• You generally don't start your own spans; the engine wraps Scan. Add spans only for expensive internal phases if useful.

Testing

Follow github_test.go / jaeger_test.go (testify: require for fatal, assert otherwise):

• A newTestConnector(t, http.HandlerFunc) helper spins up httptest.NewServer and calls New(map[string]any{"base_url": srv.URL}).
• A collectScan(conn, req) helper drives Scan with an emit that accumulates chunks, so you can assert on the full result; a scanChunkSizes variant asserts the streaming/pagination shape.
• The handler both returns canned JSON and captures the inbound request (r.URL.Path, r.URL.Query()), so you can assert push-down landed in the outbound call.

Cover at least: Tables(); a happy-path scan (row mapping, types, column order); push-down assertions; LIMIT-not-pushed when ordering can't be honored; missing-required-filter returns an error without calling the API; pagination/ streaming; and API-error surfacing. make coverage enforces a coverage gate.

Database connectors can't use httptest. Follow internal/source/postgres instead: factor the request-building into pure functions (buildSelect, type mapping, value normalization) and unit-test those directly (asserting the generated SQL + args is the analog of capturing the outbound HTTP request) so they count toward coverage; then add a gated integration test (//go:build integration) that runs against a real database from $DFETCH_TEST_POSTGRES_DSN and t.Skips when it's unset. CI provides the database as a service container.

Reference implementations

internal/source/github — REST over net/http. Shows: required path-param filters (requireStringEq), mapping ORDER BY to the API's sort param (orderParam), safe LIMIT push-down accounting for OFFSET (pageLimit, limitSafe, consumedAll), Link-header pagination (nextLink), one chunk emitted per page, and preserving the caller's owner casing so SQLite's case-sensitive WHERE still matches.

internal/source/jaeger — Jaeger api_v3 returning the OTLP model. Shows: required service_name, deriving a time window from range filters on a start_time column (timeBounds, defaulting to the last hour) and a duration window (durationBounds), a trace_id equality short-circuiting to a by-id endpoint (no service/window needed), flattening nested JSON into rows, an attributes JSON column, OTLP enums rendered as readable strings, and decoding a streamed grpc-gateway response with a json.Decoder loop (one chunk per decoded object).

internal/source/postgres — the reference dynamic / SQL connector (database/sql + jackc/pgx). Shows: an empty Tables() with SchemaDescriber/ TableLister backed by information_schema; mapping Postgres types to SQLite affinities; and translating a ScanRequest into a real parameterized SELECT … WHERE … ORDER BY … LIMIT (buildSelect) — pushing only operators Postgres and SQLite evaluate identically (no LIKE), pushing ORDER BY+LIMIT only for order-safe column types with NULL placement aligned to SQLite (orderPushSafe), and honoring the engine-supplied column projection (ScanRequest.Columns). It is config-only (registered for type: postgres, never auto-instantiated), so it has no base_url-style builtin default.

Invariants and gotchas

• Superset is fine; wrong rows are not. When unsure whether the API honors a predicate exactly, don't push it.
• Row arity and order must match the declared columns, every chunk.
• Case sensitivity: SQLite's default collation is case-sensitive and the engine runs the user's WHERE verbatim. If you store a value the user filtered on, store it as they wrote it (see github's owner handling), or the local re-filter may drop the row.
• nil for NULL, int64 for integers, bool for INTEGER 0/1.
• Required filters → error, not full fetch.
• Cap unbounded scans when LIMIT can't be pushed.
• New(nil) must work for builtins.
• No global state — connectors are constructed once and used concurrently across sources; keep Scan safe for concurrent calls (a shared *http.Client is fine).

Step-by-step

1. Create the package internal/source/<name>/<name>.go: a Connector struct (HTTP client with otelhttp transport, base URL, optional token) and a New(params) (source.Connector, error) factory reading params["base_url"] and env secrets, defaulting so New(nil) works.
2. Declare tables/columns (often in a tables.go): []source.Column per table with SQLite affinities, and Tables() returning them.
3. Implement Scan: dispatch on req.Table to a per-table scanX; build the upstream request from safe push-down; page through results and emit one *source.Rows per page with values in column order.
4. Add push-down helpers as needed (equality extraction, range→window, order/limit mapping) — copy github/jaeger patterns; only push what's safe.
5. Register in engine.go builtins (and/or document config usage).
6. Test with httptest + testify (see Testing).
7. Document: add the connector to connectors.md (and an example group in examples.yaml, rendered via make examples), and a layout line in CONTRIBUTING.md.

Checklist

• internal/source/<name>/ package with New, Tables, Scan
• New(nil) works with sensible defaults; base_url override supported
• Columns use SQLite affinities; rows match column count/order; nil/int64/bool used correctly
• Push-down only when safe; required filters error; unbounded scans capped
• One chunk emitted per page; ctx threaded; emit errors propagated
• Registered in engine.go builtins (or documented config usage)
• Tests (httptest + testify) cover tables, scan, push-down, errors; make coverage passes
• make lint and make vet clean
• README Connectors section + CONTRIBUTING layout updated