Sequences

The @fluidframework/sequence packages supports distributed data structures which are list-like. It includes SharedString for storing and simultaneously editing a sequence of text. Note that SharedString is a sequence DDS but it has additional specialized features and behaviors for working with text.

Sequence DDSes share a common base class, SharedSegmentSequence. For the remainder of this document, the term sequence will refer to this base class.

Items are the individual units that are stored within the sequence (e.g. in a SharedString, the items are characters), but regardless of the type of data stored in the sequence, every item in a sequence is at a specific position starting at 0, similar to an array. However, sequences differ from arrays in that the positions can move as local and remote editors make modifications to the sequence.

As its name suggests, SharedSegmentSequence is composed of segments. Segments are the unit that the sequence works with internally, and contain items within them. Thus, every segment has a length of at least 1 – that is, it contains at least one item – and segments may be split and merged arbitrarily as the sequence is edited. This means the length of the sequence is not the number of segments, but rather the sum of the length of all the segments.

For example, consider a SharedString that is initially empty. User A adds the characters a, b, and c to the sequence. Its length is now 3 – it contains 3 items. Internally, however, the sequence could have either 1, 2, or 3 segments.

Segments: [S1] [S2] [S3]
   Items:  a    b    c

Segments: [  S1  ]  [S2]
   Items:  a    b    c

Segments: [    S1     ]
   Items:  a    b    c

In typical use, the splitting and merging of segments is an implementation detail that is not relevant to using the sequence. However, it is possible to enumerate the segments that intersect a range of positions for performance reasons. In this case it is important to not retain references to the segments (outside of the enumeration), and to make no assumptions based on the length of the segments themselves.

Using a Sequence

Sequences support three basic operations: insert, remove, and annotate. Insert and remove are used to add and remove items from the sequence, while annotate is used to add metadata to items.

Insert

Insert operations on the sequence take a single position argument along with the content. This position is inclusive and can be any position in the sequence including 0, to insert at the beginning of the sequence, and the length of the sequence, to insert at the end.

    //   content:
    // positions:

    // insert text at position 0
    sharedString.insertText(0, "hi");
    //   content: hi
    // positions: 01

    // insert text at the end position
    sharedString.insertText(
        sharedString.getLength(),
        "!");
    //   content: hi!
    // positions: 012

    // insert text at position 2
    sharedString.insertText(
        2,
        " world");
    //   content: hi world!
    // positions: 012345678

Remove

Remove operations take a start and an end position, referred to as a range. The start position is inclusive and can be any position in the sequence from 0 to its length - 1. The start position cannot be the length of the sequence like it can in insert, because there is nothing at that position. The end position is exclusive and must be greater than the start, so it can be any value from 1 to n (where n is the length of the sequence).

    //   content: hi world!
    // positions: 012345678

    // remove the first 3 characters
    sharedString.removeRange(0, 3);
    //   content: world!
    // positions: 012345

    // remove all the characters
    sharedString.removeRange(0, sharedString.getLength());
    //   content:
    // positions:

Annotate

Annotate operations can add or remove map-like properties to or from items in the sequence. They can store any JSON serializable data and have the same merge behavior as a SharedMap (last writer wins). Annotate takes a start and end position which work the same way as the start and end of the remove operation. In addition to start and end, annotate also takes a map-like properties object. Each key of the provided properties object will be set on each position of the specified range. Setting a property key to null will remove that property from the positions in the range.

    //   content: hi world
    // positions: 01234567

    let props1 = sharedString.getPropertiesAtPosition(1);
    let props5 = sharedString.getPropertiesAtPosition(5);
    // props1 = {}
    // props5 = {}

    // set property called weight on positions 0 and 1
    sharedString.annotateRange(0, 2, { weight: 5 });
    props1 = sharedString.getPropertiesAtPosition(1);
    props5 = sharedString.getPropertiesAtPosition(5);
    // props1 = { weight: 5 }
    // props5 = {}

    // set property called decoration on all positions
    sharedString.annotateRange(
        0,
        sharedString.getLength(),
        { decoration: "underline" });
    props1 = sharedString.getPropertiesAtPosition(1);
    props5 = sharedString.getPropertiesAtPosition(5);
    // props1 = { weight: 5, decoration: "underline" }
    // props5 = { decoration: "underline" }

    // remove property called weight on all positions
    sharedString.annotateRange(
        0,
        sharedString.getLength(),
        { weight: null });
    props1 = sharedString.getPropertiesAtPosition(1);
    props5 = sharedString.getPropertiesAtPosition(5);
    // props1 = { decoration: "underline" }
    // props5 = { decoration: "underline" }

Sequence delta event

Whenever an operation is performed on a sequence a sequenceDelta event will be raised. This event provides the ranges affected by the operation, the type of the operation, and the properties that were changes by the operation.

Sequence merge strategy

The Fluid sequence data structures are eventually consistent, which means all editors will end up in the same final state. However, the intermediate states seen by each collaborator may not be seen by other collaborators. These intermediate states occur when two or more collaborators modify the same position in the sequence which results in a conflict.

Merge strategy for insert

Consider a sequence like this:

    //   content: hi mar
    // positions: 012345

Now two users simultaneously insert characters at the end of the sequence. One inserts k and the other inserts a c. This is an insert conflict. The basic strategy for insert conflict resolution in the sequence is to merge far, closer to the end of the sequence.

This merge strategy is possible because of a fundamental property of the Fluid Framework, which is guaranteed ordering. That is, while the two inserts occurred simultaneously, the operations will be given a global order and all clients will see the order of the operations when applying them locally. This enables each client to converge to the same state eventually.

In the earlier example, assuming the k operation was ordered before the c operation, then the k would be inserted at position 6 first. Then the c op is applied – this is the merge conflict. The c op is inserted at the position requested (6), and the k is pushed out towards the end of the sequence.

    //   content: hi mar
    // positions: 012345

    // insert(6, "k")
    // k op is ordered first
    //   content: hi mark
    // positions: 0123456

    // insert(6, "c")
    // c op is now applied, pushing the k towards the end of the sequence
    //   content: hi marck
    // positions: 01234567

This same logic applies if multiple items are inserted at the same position – the earlier ordered items will be pushed towards the end of the sequence as the later items are merged.

Merge strategies for remove

Like insert, the strategies for remove and annotate also use the guaranteed ordering provided by the Fluid Framework. Consider again the example from above. Now one user inserts a y at position 6, and another user removes the c and the k (positions 6 and 7).

    //   content: hi marck
    // positions: 01234567

    // REMOVE BEFORE INSERT
    // remove(6, 7)
    // remove op now applied
    //   content: hi mar
    // positions: 012345

    // insert(6, "y")
    // no merge conflict -- position 6 is empty
    //   content: hi mary
    // positions: 0123456

    // OR

    // INSERT BEFORE REMOVE
    // insert(6, "y")
    // y op is now applied, pushing the c and k towards the end of the sequence
    //   content: hi maryck
    // positions: 012345678

    // remove(6, 7)
    // remove op now applied, but only removes content ordered before it
    //   content: hi mary
    // positions: 0123456

The key to this merge behavior is that a remove operation will only remove content that was visible to it when the operation was made. In the example above, the remove op adjusted the range it removed, ensuring only the ck was removed.

Another way to consider this behavior is that a remove operation will only remove content that was inserted earlier in the order. Anything inserted after a remove operation will be ignored. The sequence also detects overlapping remove operations, and the merge resolution is straightforward – the data is removed.

Merge strategy for annotate

As mentioned above, annotate operations behave like operations on SharedMaps. The merge strategy used is last writer wins. If two collaborators set the same key on the annotate properties the operation that gets ordered last will determine the value.

SharedString

The SharedString is a specialized data structure for handling collaborative text. It is based on a more general Sequence data structure but has additional features that make working with text easier.

In addition to text, a SharedString can also contain markers. Markers can be used to store metadata at positions within the text, like the details of an image or Fluid object that should be rendered with the text.

Both markers and text are stored as segments in the SharedString. Text segments will be split and merged when modifications are made to the SharedString and will therefore have variable length matching the length of the text content they contain. Marker segments are never split or merged, and always have a length of 1.

The length of the SharedString will be the combined length of all the text and marker segments. Just like with other sequences, when talking about positions in a SharedString we use the terms near and far. The nearest position in a SharedString is 0, and the farthest position is its length. When comparing two positions the nearer positions is closer to 0, and the farther position is closer to the length.