Documentation/technical/reftable: improve repo layout
Previously, the list of tables was in .git/refs. This makes repo detection fail in older clients, which is undesirable. This is proposal was discussed and approved on the git@vger list at https://lore.kernel.org/git/CAFQ2z_PvKiz==GyS6J1H1uG0FRPL86JvDj+LjX1We4-yCSVQ+g@mail.gmail.com/ For backward compatibility, JGit could detect a file under .git/refs and use it as a reftable list. Change-Id: Ic0b974fa250cfa905463b811957e2a4fdd7bbc6b Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
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@ -773,9 +773,6 @@ A repository must set its `$GIT_DIR/config` to configure reftable:
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### Layout
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### Layout
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The `$GIT_DIR/refs` path is a file when reftable is configured, not a
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directory. This prevents loose references from being stored.
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A collection of reftable files are stored in the `$GIT_DIR/reftable/`
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A collection of reftable files are stored in the `$GIT_DIR/reftable/`
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directory:
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directory:
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@ -789,28 +786,38 @@ the function `${min_update_index}-${max_update_index}.ref`.
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Log-only files use the `.log` extension, while ref-only and mixed ref
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Log-only files use the `.log` extension, while ref-only and mixed ref
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and log files use `.ref`. extension.
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and log files use `.ref`. extension.
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The stack ordering file is `$GIT_DIR/refs` and lists the current
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files, one per line, in order, from oldest (base) to newest (most
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recent):
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$ cat .git/refs
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The stack ordering file is `$GIT_DIR/reftable/tables.list` and lists the current
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files, one per line, in order, from oldest (base) to newest (most recent):
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$ cat .git/reftable/tables.list
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00000001-00000001.log
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00000001-00000001.log
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00000002-00000002.ref
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00000002-00000002.ref
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00000003-00000003.ref
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00000003-00000003.ref
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Readers must read `$GIT_DIR/refs` to determine which files are
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Readers must read `$GIT_DIR/reftable/tables.list` to determine which files are
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relevant right now, and search through the stack in reverse order
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relevant right now, and search through the stack in reverse order (last reftable
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(last reftable is examined first).
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is examined first).
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Reftable files not listed in `refs` may be new (and about to be added
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Reftable files not listed in `tables.list` may be new (and about to be added
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to the stack by the active writer), or ancient and ready to be pruned.
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to the stack by the active writer), or ancient and ready to be pruned.
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### Backward compatibility
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Older clients should continue to recognize the directory as a git repository so
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they don't look for an enclosing repository in parent directories. To this end,
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a reftable-enabled repository must contain the following dummy files
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* `.git/HEAD`, a regular file containing `ref: refs/heads/.invalid`.
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* `.git/refs/`, a directory
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* `.git/refs/heads`, a regular file
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### Readers
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### Readers
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Readers can obtain a consistent snapshot of the reference space by
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Readers can obtain a consistent snapshot of the reference space by
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following:
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following:
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1. Open and read the `refs` file.
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1. Open and read the `tables.list` file.
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2. Open each of the reftable files that it mentions.
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2. Open each of the reftable files that it mentions.
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3. If any of the files is missing, goto 1.
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3. If any of the files is missing, goto 1.
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4. Read from the now-open files as long as necessary.
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4. Read from the now-open files as long as necessary.
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@ -820,13 +827,13 @@ following:
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Although reftables are immutable, mutations are supported by writing a
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Although reftables are immutable, mutations are supported by writing a
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new reftable and atomically appending it to the stack:
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new reftable and atomically appending it to the stack:
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1. Acquire `refs.lock`.
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1. Acquire `tables.list.lock`.
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2. Read `refs` to determine current reftables.
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2. Read `tables.list` to determine current reftables.
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3. Select `update_index` to be most recent file's `max_update_index + 1`.
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3. Select `update_index` to be most recent file's `max_update_index + 1`.
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4. Prepare temp reftable `tmp_XXXXXX`, including log entries.
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4. Prepare temp reftable `tmp_XXXXXX`, including log entries.
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5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}.ref`.
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5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}.ref`.
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6. Copy `refs` to `refs.lock`, appending file from (5).
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6. Copy `tables.list` to `tables.list.lock`, appending file from (5).
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7. Rename `refs.lock` to `refs`.
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7. Rename `tables.list.lock` to `tables.list`.
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During step 4 the new file's `min_update_index` and `max_update_index`
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During step 4 the new file's `min_update_index` and `max_update_index`
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are both set to the `update_index` selected by step 3. All log
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are both set to the `update_index` selected by step 3. All log
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@ -834,9 +841,9 @@ records for the transaction use the same `update_index` in their keys.
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This enables later correlation of which references were updated by the
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This enables later correlation of which references were updated by the
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same transaction.
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same transaction.
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Because a single `refs.lock` file is used to manage locking, the
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Because a single `tables.list.lock` file is used to manage locking, the
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repository is single-threaded for writers. Writers may have to
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repository is single-threaded for writers. Writers may have to
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busy-spin (with backoff) around creating `refs.lock`, for up to an
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busy-spin (with backoff) around creating `tables.list.lock`, for up to an
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acceptable wait period, aborting if the repository is too busy to
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acceptable wait period, aborting if the repository is too busy to
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mutate. Application servers wrapped around repositories (e.g. Gerrit
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mutate. Application servers wrapped around repositories (e.g. Gerrit
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Code Review) can layer their own lock/wait queue to improve fairness
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Code Review) can layer their own lock/wait queue to improve fairness
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@ -864,21 +871,21 @@ For sake of illustration, assume the stack currently consists of
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reftable files (from oldest to newest): A, B, C, and D. The compactor
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reftable files (from oldest to newest): A, B, C, and D. The compactor
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is going to compact B and C, leaving A and D alone.
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is going to compact B and C, leaving A and D alone.
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1. Obtain lock `refs.lock` and read the `refs` file.
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1. Obtain lock `tables.list.lock` and read the `tables.list` file.
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2. Obtain locks `B.lock` and `C.lock`.
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2. Obtain locks `B.lock` and `C.lock`.
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Ownership of these locks prevents other processes from trying
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Ownership of these locks prevents other processes from trying
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to compact these files.
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to compact these files.
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3. Release `refs.lock`.
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3. Release `tables.list.lock`.
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4. Compact `B` and `C` into a temp file `${min_update_index}-${max_update_index}_XXXXXX`.
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4. Compact `B` and `C` into a temp file `${min_update_index}-${max_update_index}_XXXXXX`.
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5. Reacquire lock `refs.lock`.
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5. Reacquire lock `tables.list.lock`.
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6. Verify that `B` and `C` are still in the stack, in that order. This
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6. Verify that `B` and `C` are still in the stack, in that order. This
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should always be the case, assuming that other processes are adhering
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should always be the case, assuming that other processes are adhering
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to the locking protocol.
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to the locking protocol.
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7. Rename `${min_update_index}-${max_update_index}_XXXXXX` to
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7. Rename `${min_update_index}-${max_update_index}_XXXXXX` to
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`${min_update_index}-${max_update_index}.ref`.
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`${min_update_index}-${max_update_index}.ref`.
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8. Write the new stack to `refs.lock`, replacing `B` and `C` with the
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8. Write the new stack to `tables.list.lock`, replacing `B` and `C` with the
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file from (4).
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file from (4).
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9. Rename `refs.lock` to `refs`.
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9. Rename `tables.list.lock` to `tables.list`.
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10. Delete `B` and `C`, perhaps after a short sleep to avoid forcing
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10. Delete `B` and `C`, perhaps after a short sleep to avoid forcing
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readers to backtrack.
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readers to backtrack.
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