# Sync modes > The five ways sanda can read a source and write it to your warehouse, with what each needs and how each treats edits, deletes and history. A sync mode is set per stream. It says how sanda reads the table at the source and how it writes the table in your warehouse. For advice on choosing, see [Sync modes](https://docs.sanda-os.com.au/connections/sync-modes) in the connections guide. ## The modes at a glance | Mode | Needs a cursor | Needs a primary key | What it does | | --- | --- | --- | --- | | **Full refresh · Overwrite** (default) | No | No | Reads the whole table on every run and replaces what landed last time. | | **Full refresh · Overwrite + Deduped** | No | Yes | Reads the whole table on every run, replaces what landed and keeps one row per primary key. | | **Full refresh · Append** | No | No | Reads the whole table on every run and adds it beside the earlier copies: a history of snapshots. | | **Incremental · Append** | Yes | No | Reads only the rows whose cursor has moved since the last run, and adds them. | | **Incremental · Append + Deduped** | Yes | Yes | Reads only the rows whose cursor has moved, and keeps the latest version of each primary key. | A **cursor** is a column that moves forward whenever a row is created or changed, such as `updated_at`. A **primary key** is the column, or columns, that identify one row. Both are chosen in the [stream picker](https://docs.sanda-os.com.au/connections/choose-streams) unless the source has already decided them. ## Full refresh · Overwrite **Shown in stream lists as** `full refresh`. **Needs** nothing. **This is the default.** Reads the whole table on every run and replaces what landed last time. - **Edits at the source:** the table shows the new value after the next run. - **Deletes at the source:** the row is gone after the next run, because the whole table is replaced. - **History:** none. Only the result of the latest run exists. It is the default for any stream you have not chosen a mode for, because every table supports it and it needs nothing answered. Where a table does not offer it, the picker opens on the first mode the table does offer. ## Full refresh · Overwrite + Deduped **Shown in stream lists as** `full refresh · dedup`. **Needs** a primary key. Reads the whole table on every run, replaces what landed, and keeps one row per primary key. - **Edits at the source:** the table shows the new value after the next run. - **Deletes at the source:** the row is gone after the next run. - **History:** none. Use it when a source can return the same key more than once and you want a single row for each. ## Full refresh · Append **Shown in stream lists as** `full refresh · append`. **Needs** nothing. Reads the whole table on every run and adds it beside the earlier copies: a history of snapshots. - **Edits at the source:** each run's copy holds the value the row had at that run. - **Deletes at the source:** the earlier copies still hold the row. It is missing from the copies that follow. - **History:** one full copy per run. The table grows by the size of the source on every run, and the storage counts toward your warehouse limit. Each landed row carries a loading column, whose name starts with an underscore, that records when it was extracted. It is what tells one run's copy from the next. ## Incremental · Append **Shown in stream lists as** `incremental`. **Needs** a cursor. Reads only the rows whose cursor has moved since the last run, and adds them. The first run has nothing to compare with, so it reads the whole table. - **Edits at the source:** the changed row lands again as a new row, and the earlier version stays. - **Deletes at the source:** a cursor cannot see a row that is no longer there, so the row stays in the table. The exception is a source read by its database's change log, which reports a deletion as a flag on the row. - **History:** every version of every row that a run has seen. ## Incremental · Append + Deduped **Shown in stream lists as** `incremental · dedup`. **Needs** a cursor and a primary key. Reads only the rows whose cursor has moved, and keeps the latest version of each primary key. - **Edits at the source:** the table holds the latest version. - **Deletes at the source:** the row stays, for the same reason as **Incremental · Append**. - **History:** the table holds one row per key, not every version. ## What a mode needs | Need | Modes that ask for it | Where it comes from | |---|---|---| | A cursor | **Incremental · Append**, **Incremental · Append + Deduped** | You pick a column. The picker marks the source's suggestion. A source can also decide the cursor itself, and then the cell reads **source-defined** | | A primary key | **Full refresh · Overwrite + Deduped**, **Incremental · Append + Deduped** | You pick a column. A source can declare the key itself, and then the cell shows it | The picker refuses to save while a stream that is on lacks what its mode needs, and says how many streams are missing a cursor or a key. ## Modes a source offers Each table offers only the modes the source supports for it. A table that supports only one shows it as fixed text. When a source reports nothing about a table, all five are offered. A mode applies from the next sync after you save it. Changing a mode does not run anything by itself. ## Older connections Before sanda offered all five modes, a stream was either "full refresh" or "incremental", and sanda chose how to write it. A stream saved that way keeps doing what it did: a full refresh overwrote, and an incremental deduplicated when the table had a key (its own or the source's) and appended otherwise. Open **Edit streams** to move it to one of the five. ## What is the same in every mode - The table lands as `raw.__`, whichever mode reads it. - Every landed table carries a few loading columns whose names start with an underscore. They stay out of the semantic fluid. - A run that replaces a table is followed by a short window, until sanda has attached the new copy, in which a question over the table returns a message that it has no rows attached right now. See [Troubleshoot connections](https://docs.sanda-os.com.au/connections/troubleshooting#a-table-has-no-rows-attached-right-now). - Each successful run is metered as one run, however many rows it moves. :::links - [Sync modes](https://docs.sanda-os.com.au/connections/sync-modes): How to choose a mode for each table. - [Choose what to sync](https://docs.sanda-os.com.au/connections/choose-streams): The picker, cursors and primary keys. :::