SSTables 3.0 Statistics File Format

New in version 3.0.

This file stores metadata for SSTable. There are 4 types of metadata:

  1. Validation metadata - used to validate SSTable correctness.

  2. Compaction metadata - used for compaction.

  3. Statistics - some information about SSTable which is loaded into memory and used for faster reads/compactions.

  4. Serialization header - keeps information about SSTable schema.

General structure

The file is composed of two parts. First part is a table of content which allows quick access to a selected metadata. Second part is a sequence of metadata stored one after the other. Let’s define array template that will be used in this document.

struct array<LengthType, ElementType> {
    LengthType number_of_elements;
    ElementType elements[number_of_elements];
}

Table of content

using toc = array<be32<int32_t>, toc_entry>;

struct toc_entry {
    // Type of metadata
    // | Type                 | Integer representation |
    // |----------------------|------------------------|
    // | Validation metadata  | 0                      |
    // | Compaction metadata  | 1                      |
    // | Statistics           | 2                      |
    // | Serialization header | 3                      |
    be32<int32_t> type;
    // Offset, in the file, at which this metadata entry starts
    be32<int32_t> offset;
}

The toc array is sorted by the type field of its members.

Validation metadata entry

struct validation_metadata {
    // Name of partitioner used to create this SSTable.
    // Represented by UTF8 string encoded using modified UTF-8 encoding.
    // You can read more about this encoding in:
    // https://docs.oracle.com/javase/7/docs/api/java/io/DataInput.html#modified-utf-8
    // https://docs.oracle.com/javase/7/docs/api/java/io/DataInput.html#readUTF()
    Modified_UTF-8_String partitioner_name;
    // The probability of false positive matches in the bloom filter for this SSTable
    be64<double> bloom_filter_fp_chance;
}

Compaction metadata entry

// Serialized HyperLogLogPlus which can be used to estimate the number of partition keys in the SSTable.
// If this is not present then the same estimation can be computed using Summary file.
// Encoding is described in:
// https://github.com/addthis/stream-lib/blob/master/src/main/java/com/clearspring/analytics/stream/cardinality/HyperLogLogPlus.java
using compaction_metadata = array<be32<int32_t>, be8>;

Statistics entry

This entry contains some parts of EstimatedHistogram, StreamingHistogram and CommitLogPosition types. Let’s have a look at them first.

EstimatedHistogram

// Each bucket represents values from (previous bucket offset, current offset].
// Offset for last bucket is +inf.
using estimated_histogram = array<be32<int32_t>, bucket>;

struct bucket {
    // Offset of the previous bucket
    // In the first bucket this is offset of the first bucket itself because there's no previous bucket.
    // The offset of the first bucket is repeated in second bucket as well.
    be64<int64_t> prev_bucket_offset;
    // This bucket value
    be64<int64_t> value;
}

StreamingHistogram

struct streaming_histogram {
    // Maximum number of buckets this historgam can have
    be32<int32_t> bucket_number_limit;
    array<be32<int32_t>, bucket> buckets;
}

struct bucket {
    // Offset of this bucket
    be64<double> offset;
    // Bucket value
    be64<int64_t> value;
}

CommitLogPosition

struct commit_log_position {
    be64<int64_t> segment_id;
    be32<int32_t> position_in_segment;
}

Whole entry

struct statistics {
    // In bytes, uncompressed sizes of partitions
    estimated_histogram partition_sizes;
    // Number of cells per partition
    estimated_histogram column_counts;
    commit_log_position commit_log_upper_bound;
    // Typically in microseconds since the unix epoch, although this is not enforced
    be64<int64_t> min_timestamp;
    // Typically in microseconds since the unix epoch, although this is not enforced
    be64<int64_t> max_timestamp;
    // In seconds since the unix epoch
    be32<int32_t> min_local_deletion_time;
    // In seconds since the unix epoch
    be32<int32_t> max_local_deletion_time;
    be32<int32_t> min_ttl;
    be32<int32_t> max_ttl;
    // compressed_size / uncompressed_size
    be64<double> compression_rate;
    // Histogram of cell tombstones.
    // Keys are local deletion times of tombstones
    streaming_histogram tombstones;
    be32<int32_t> level;
    // The difference, measured in milliseconds, between repair time and midnight, January 1, 1970 UTC
    be64<int64_t> repaired_at;
    clustering_bound min_clustering_key;
    clustering_bound max_clustering_key;
    be8<bool> has_legacy_counters;
    be64<int64_t> number_of_columns;
    be64<int64_t> number_of_rows;

    // Version MA of SSTable 3.x format ends here.
    // It contains only one commit log position interval - [NONE = new CommitLogPosition(-1, 0), upper bound of commit log]

    commit_log_position commit_log_lower_bound;

    // Version MB of SSTable 3.x format ends here.
    // It contains only one commit log position interval - [lower bound of commit log, upper bound of commit log].

    array<be32<int32_t>, commit_log_interval> commit_log_intervals;
}

using clustering_bound = array<be32<int32_t>, clustering_column>;
using clustering_column = array<be16<uint16_t>, be8>;

struct commit_log_interval {
    commit_log_position start;
    commit_log_position end;
}

Serialization header

struct serialization_header {
    vint<uint64_t> min_timestamp;
    vint<uint32_t> min_local_deletion_time;
    vint<uint32_t> min_ttl;
    // If partition key has one column then this is the type of this column.
    // Otherwise, this is a CompositeType that contains types of all partition key columns.
    type partition_key_type;
    array<vint<uint32_t>, type> clustering_key_types;
    columns static_columns;
    columns regular_columns;
}

using columns = array<vint<uint32_t>, column>;

struct column {
    array<vint<uint32_t>, be8> name;
    type column_type;
}

// UTF-8 string
using type = array<vint<uint_32_t>, be8>;

Type encoding

Type is just a byte buffer with an unsigned variant integer (32-bit) length. It is a UTF-8 string. All leading spaces, tabs and newlines are skipped. Null or empty string is a bytes type. First segment of non-blank characters should contain only alphanumerical characters and special chars like '-', '+', '.', '_', '&'. This is the name of the type. If type name does not contain any ‘.’ then it gets “org.apache.cassandra.db.marshal.” prepended to itself. Then an “instance” static field is taken from this class. If the first non-blank character that follows type name is ‘(‘ then “getInstance” static method is invoked instead. Remaining string is passed to this method as a parameter. There are following types:

Type

Parametrized

Ascii Type

No

Boolean Type

No

Bytes Type

No

Byte Type

No

ColumnToCollection Type

Yes

Composite Type

Yes

CounterColumn Type

No

Date Type

No

Decimal Type

No

Double Type

No

Duration Type

No

DynamicComposite Type

Yes

Empty Type

No

Float Type

No

Frozen Type

Yes

InetAddress Type

No

Int32 Type

No

Integer Type

No

LexicalUUID Type

No

List Type

Yes

Long Type

No

Map Type

Yes

PartitionerDefinedOrder

Yes

Reversed Type

Yes

Set Type

Yes

Short Type

No

SimpleDate Type

No

Timestamp Type

No

Time Type

No

TimeUUID Type

No

Tuple Type

Yes

User Type

Yes

UTF8 Type

No

UUID Type

No

Copyright

© 2016, The Apache Software Foundation.

Apache®, Apache Cassandra®, Cassandra®, the Apache feather logo and the Apache Cassandra® Eye logo are either registered trademarks or trademarks of the Apache Software Foundation in the United States and/or other countries. No endorsement by The Apache Software Foundation is implied by the use of these marks.