---
title: "External Attribute Sync"
description: "External Attribute Sync projects selected Flow Attribute writes into PostgreSQL"
canonical_url: "https://docs.superdurable.io/primitives/components/external-attribute-sync/"
source: "docs/content/primitives/components/external-attribute-sync.mdx"
locale: "en"
---

# External Attribute Sync

External Attribute Sync projects selected Flow Attribute writes into PostgreSQL
tables that you own. Each destination is called an Attribute Store.
Dex uses asynchronous, latest-state writes for one Flow at a time. Delivery is
attempted at least once, so every destination write is idempotent.

PostgreSQL is the only supported Attribute Store. Redis support is tracked in
[issue 616](https://github.com/superdurable/dex/issues/616).

Dex never creates, migrates, or backfills a destination. Create the SQL table
before starting the server.

## How it fits with the other components

External Attribute Sync is a [server](https://docs.superdurable.io/primitives/components/server.md) feature. A
Flow marks an Attribute with **syncToAttributeStore** and lists a configured
store name in its **FlowConfig**. server-zd then writes the Attribute to that
store's tables. Your [application](https://docs.superdurable.io/primitives/components/application.md) and your
own reports query those tables with SQL.

## Configuration

```yaml
attributeStore:
  stores:
    reporting:
      type: postgres
      dsn: postgres://user:password@host/database
      tableName: public.flow_attributes
    orders:
      type: postgres
      dsn: postgres://user:password@host/database
      tableName: public.orders
      rowKey: order-{customer_id}-{order_id}
      columnNaming: snake_case
      attributeMaps:
        LineItems:
          tableName: public.order_line_items
          instanceKeyColumn: line_item_id
  schemaSyncInterval: 1m
  syncBatchSize: 100
  syncAttemptTimeout: 30s
  syncRetryPolicy:
    initialInterval: 100ms
    maximumInterval: 30s
    backoffCoefficient: 2
    maximumAttempts: 0
    totalDuration: 1h
```

Every store requires **type: postgres**, **tableName**, and a DSN, which is
**dsn** or an environment variable. server-zd never logs DSNs. **tableName** accepts **table** or **schema.table**.
Identifiers are parsed and quoted. Attribute values always use driver
parameters. See
[Attribute Store sync configuration](https://docs.superdurable.io/reference/server/attribute-store-sync.md#attribute-store-configuration)
for every key and its default.

## Delivery and retries

Attribute Sync stores latest state, not an event history. A later write may
repeat after an ambiguous failure. Upserts therefore use the Flow ID as their
idempotency key.

Writes never block a Step, RPC, or SetAttributes commit. Failed writes are
retried until **syncRetryPolicy.totalDuration**, then dropped with an error log.
Dex does not automatically replay old Attributes or backfill a newly configured
store. Filtered writes, missing targets, and retry exhaustion emit error logs.

See [Attribute Store sync](https://docs.superdurable.io/reference/server/attribute-store-sync.md) in the
Server reference for the fencing table, closing behavior, retries, metrics, row
keys, AttributeMap tables, and the SQL type contract.

<a id="attribute-store-local-development"></a>

## Local development

**dex-server-zd dev** runs one local node with embedded object storage. Its
**--attribute-store-config** flag reads only the **attributeStore** section of a
YAML file. That section must define at least one store. The
**--config** overrides file must not also set **attributeStore**.

```bash
dex-server-zd dev --data-dir ./zd-data --attribute-store-config ./attribute-store.yaml
```

The database still needs its tables and the fencing table.
**dexcli dev --attribute-store-config** passes the file to this
flag.