# File format of the Scylla.db sstable component The `Scylla.db` component (present in a file named like `mc-223-big-Scylla.db` contains assorted Scylla-only metadata. Its presence indicates the sstable was created by Scylla (or some Scylla-aware creator). Non-Scylla consumers will ignore it. The file is small and intended to be processed in-memory. ## Main structure The main structure is that of an unordered set of subcomponents. Each component is prefixed with a be32 tag that indicates its type, and its serialized size (so unknown subcomponents can be skipped). scylla_db = subcomponent_count (tag serialized_size subcomponent)* subcomponent_count = be32 serialized_size = be32 tag = be32 ## Subcomponents and tag values The following subcomponents are recognized. They are described in more detail in individual sections subcomponent = sharding_metadata | features | extension_attributes | run_identifier | large_data_stats | sstable_origin | scylla_build_id | scylla_version | ext_timestamp_stats | schema | components_digests | large_data_records `sharding_metadata` (tag 1): describes what token sub-ranges are included in this sstable. This is used, when loading the sstable, to determine which shard(s) it occupies. `features` (tag 2): a set of boolean flags that describe the sstable `extension_attributes` (tag 3): a `map` with additional attributes `run_identifier` (tag 4): a uuid that is the same for all sstables in the same run (and different for sstables in different runs). `large_data_stats` (tag 5): a `map` with statistics about large data entities in the sstable. `sstable_origin` (tag 6): a string describing the origin of the sstable ("memtable" for memtable flush, "garbage collection" for compaction, etc.). `scylla_build_id` (tag 7): a string containing the build id of the Scylla executable that created the sstable. `scylla_version` (tag 8): a string containing the version of the Scylla executable that created the sstable. `ext_timestamp_stats` (tag 9): a `map` with statistics about timestamps in the sstable, like: `min_live_timestamp`, and `min_live_row_marker_timestamp`. `sstable_identifier` (tag 10): a uuid identifying the sstable for its whole lifetime. It is derived from the sstable uuid generation, upon creation (or uniquely generated if the sstable has numerical generation). Yet, unlike the sstable that may change if the sstable is migrated to a different shard or node, the sstable identifier is stable and copied with the rest of the scylla metadata. `schema` (tag 11): the schema of the table the sstable belongs to. It stores the most important fields: the table id and version (as UUIDs), keyspace name, table name, and a list of all columns with their kind, name and type. It is not a complete schema equivalent to the one stored in the system schema tables, but it contains enough information for tools like [scylla-sstable](https://github.com/scylladb/scylladb/blob/master/docs/operating-scylla/admin-tools/scylla-sstable.rst) to parse an sstable in a self-sufficient manner. `components_digests` (tag 12): a `map` with CRC32 digests of all SSTable component files that are checksummed during write. Each entry maps a component type (e.g., Data, Index, Filter, Statistics, etc.) to its CRC32 checksum. This allows verifying the integrity of individual component files. `large_data_records` (tag 13): an `array` with the top-N individual large data entries (partitions, rows, cells) found during the sstable write. Unlike `large_data_stats` which only stores aggregate statistics, this records the actual keys and sizes so they survive tablet/shard migration. The [scylla sstable dump-scylla-metadata](https://github.com/scylladb/scylladb/blob/master/docs/operating-scylla/admin-tools/scylla-sstable.rst#dump-scylla-metadata) tool can be used to dump the scylla metadata in JSON format. ## Trailing digest When the `components_digests` subcomponent is present, the `Scylla.db` file contains a trailing CRC32 digest appended after the serialized subcomponents data. This digest covers the entire serialized `data` section (i.e., all subcomponents) and can be used to verify the integrity of the scylla metadata itself. ## sharding_metadata subcomponent sharding_metadata = token_range_count token_range* token_range_count = be32 token_range = left_token_bound right_token_bound left_token_bound = token_bound right_token_bound = token_bound token_bound = exclusive_flag token exclusive_flag = byte // 0=inclusive, 1=exclusive token = token_size byte* token_size = be16 Sharding metadata is a sorted list of disjoint token ranges. Each token range consists of a left bound and a right bound; either bound may be inclusive or exclusive. The tokens are interpreted according to the partitioner. The sstable contains no partitions whose token is outside the ranges described by sharding_metadata. ## features subcomponent features = be64 // interpreted as a set of bits bit 0: NonCompoundPIEntries (if set, indicates the sstable was generated by Scylla with issue #2993 fixed) bit 1: NonCompoundRangeTombstones (if set, indicates the sstable was generated by Scylla with issue #2986 fixed) bit 2: ShadowableTombstones (if set, indicates the sstable was generated by Scylla with issue #3885 fixed) bit 3: CorrectStaticCompact (if set, indicates the sstable was generated by Scylla with issue #4139 fixed) bit 4: CorrectEmptyCounters (if set, indicates the sstable was generated by Scylla with issue #4363 fixed) bit 5: CorrectUDTsInCollections (if set, indicates that the sstable was generated by Scylla with issue #6130 fixed) bit 6: CorrectLastPiBlockWidth (if set, indicates that the width of the last promoted index block never includes the partition end marker) ## extension_attributes subcomponent extension_attributes = extension_attribute_count extension_attribute* extension_attribute_count = be32 extension_attribute = extension_attribute_key extension_attribute_value extension_attribute_key = string32 extension_attribute_value = string32 string32 = string32_size byte* string32_size = be32 There are currently no defined attributes. ## run_identifier subcomponent run_identifier = uuid uuid = uuid_high_bits uuid_low_bits uuid_high_bits = be64 uuid_low_bits = be64 If the run_identifier subcomponent is present, the sstable is part of a run. All sstables with the same run_identifier belong to the same run. They are guaranteed to be disjoint (non-overlapping) in their partition keys. ## large_data_stats subcomponent large_data_stats = large_data_count large_data_pair* large_data_count = be32 large_data_pair = large_data_type large_data_stats_entry large_data_type = partition_size | row_size | cell_size | rows_in_partition | elements_in_collection partition_size = be32(1) // partition size, in bytes row_size = be32(2) // row size, in bytes cell_size = be32(3) // cell size, in bytes rows_in_partition = be32(4) // number of rows in a partition elements_in_collection = be32(5) // number of elements in a collection large_data_stats_entry = max_value threshold above_threshold max_value = be64 threshold = be64 above_threshold = be32 The large_data_stats component holds statistics about partition, row, and cell sizes and about number of rows in partition. For each entry, it keeps the largest value for the entry type, the respective large_data threshold and the number of entities that are above the threshold. ## schema subcomponent schema = table_id table_schema_version keyspace_name table_name column_count column_description* table_id = uuid table_schema_version = uuid uuid = uuid_high_bits uuid_low_bits uuid_high_bits = be64 uuid_low_bits = be64 keyspace_name = string32 table_name = string32 column_count = be32 column_description = column_kind column_name column_type column_kind = byte // 1=partition_key, 2=clustering_key, 3=static_column, 4=regular_column column_name = string32 column_type = string32 // CQL type name (e.g. "org.apache.cassandra.db.marshal.UTF8Type") string32 = string32_size byte* string32_size = be32 The schema subcomponent stores the most important schema fields of the table the sstable belongs to. It serves as an alternative schema source to the one stored in the statistics component, which lacks column names and other metadata. Unlike the full schema stored in the system schema tables, it is not intended to be comprehensive, but it contains enough information for tools like scylla-sstable to parse an sstable in a self-sufficient manner. ## large_data_records subcomponent large_data_records = record_count large_data_record* record_count = be32 large_data_record = large_data_type partition_key clustering_key column_name value elements_count range_tombstones dead_rows large_data_type = be32 // same enum as in large_data_stats partition_key = string32 // binary serialized partition key (sstables::key::get_bytes()) clustering_key = string32 // binary serialized clustering key (clustering_key_prefix::representation()), empty if N/A column_name = string32 // column name as text, empty for partition/row entries value = be64 // size in bytes (partition, row, or cell size depending on type) elements_count = be64 // type-dependent element count (see below) range_tombstones = be64 // number of range tombstones (partition_size records only, 0 otherwise) dead_rows = be64 // number of dead rows (partition_size records only, 0 otherwise) string32 = string32_size byte* string32_size = be32 The large_data_records component holds individual top-N large data entries (partitions, rows, cells) found during the sstable write. Unlike large_data_stats, which only stores aggregate per-type statistics (max value, threshold, count above threshold), large_data_records preserves the actual partition key, clustering key, column name, and size for each above-threshold entry. This information is embedded in the sstable file itself and therefore survives tablet/shard migration. The elements_count field carries a type-dependent element count: - For partition_size and rows_in_partition records: number of rows in the partition - For cell_size and elements_in_collection records: number of elements in the collection (0 for non-collection cells) - For row_size records: 0 The range_tombstones and dead_rows fields are meaningful only for partition_size records and are zero for all other record types.