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Sequences

The @fluidframework/sequence package supports distributed data structures which are list-like. Its main export is SharedString, a DDS 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.

This package historically contained several other sequence-based DDSes, but because they have unintuitive behaviors, they are deprecated and being moved to the experimental folder.

The main reason for this is the lack of move semantics within the sequence, which becomes crucial when dealing with sequences of complex content. For that reason, all of the examples in this README use SharedString. However, the APIs discussed are available on the 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. Notably, sequences do not support a notion of "moving" a range of content.

If "move" semantics are a hard requirement for your scenario, this github issue outlines some reasonable alternatives.

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 changed by the operation.

sharedString.on("sequenceDelta", ({ deltaOperation, ranges, isLocal }) => {
	if (isLocal) {
		// undo-redo implementations frequently will only concern themselves with local ops: only operations submitted
		// by the local client should be undoable by the current user
		addOperationToUndoStack(deltaOperation, ranges);
	}

	if (deltaOperation === MergeTreeDeltaType.INSERT) {
		syncInsertSegmentToModel(deltaOperation, ranges);
	}

	// realistic app code would likely handle the other deltaOperation types as well here.
});

Internally, the sequence package depends on @fluidframework/merge-tree, and also raises MergeTreeMaintenance events on that tree as maintenance events. These events don't correspond directly to APIs invoked on a sequence DDS, but may be useful for advanced users. Maintenance events are raised whenever the underlying structure of the merge-tree changes (segments are merged, split, unlinked, etc), so applications attempting to synchronize a data model dependent on the segment structure of merge-tree should look into the semantics of these events; see MergeTreeMaintenanceType.

Both sequenceDelta and maintenance events are commonly used to synchronize or invalidate a view an application might have over a backing sequence DDS.

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.

Local references​

Sequences support addition and manipulation of local references to locally track positions in the sequence over time. As the name suggests, any created references will only exist locally; other clients will not see them. This can be used to implement user interactions with sequence data in a way that is robust to concurrent editing. For example, consider a text editor which tracks a user's cursor state. The application can store a local reference to the character after the cursor position:

//   content: hi world!
// positions: 012345678
const { segment, offset } = sharedString.getContainingSegment(5);
const cursor = sharedString.createLocalReferencePosition(
	segment,
	offset,
	ReferenceType.SlideOnRemove,
	/* any additional properties */ { cursorColor: "blue" },
);

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

// ... in some view code, retrieve the position of the local reference for rendering:
const pos = sharedString.localReferencePositionToPosition(cursor); // 5

// meanwhile, some other client submits an edit which gets applied to our string:
otherSharedString.replaceText(1, 2, "ello");

// The local sharedString state will now look like this:
//    cursor:         x
//   content: hello world!
// positions: 0123456789AB (hex)

// ... in some view code, retrieve the position of the local reference for rendering:
const pos = sharedString.localReferencePositionToPosition(cursor); // 8

Notice that even though another client concurrently edited the string, the local reference representing the cursor is still in the correct location with no further work for the API consumer. The ReferenceType.SlideOnRemove parameter changes what happens when the segment that reference is associated with is removed. SlideOnRemove instructs the sequence to attempt to slide the reference to the start of the next furthest segment, or if no such segment exists (i.e. the end of the string has been removed), the end of the next nearest one.

The webflow example demonstrates this idea in more detail.

Unlike segments, it is safe to persist local references in auxiliary data structures, such as an undo-redo stack.

Interval collections​

Sequences support creation of interval collections, an auxiliary collection of intervals associated with positions in the sequence. Like segments, intervals support adding arbitrary properties, including handles (references) to other DDSes. The interval collection implementation uses local references, and so benefits from all of the robustness to concurrent editing described in the previous section. Unlike local references, operations on interval collections are sent to all clients and updated in an eventually consistent way. This makes them suitable for implementing features like comment threads on a text-based documents. The following example illustrates these properties and highlights the major APIs supported by IntervalCollection.

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

const comments = sharedString.getIntervalCollection("comments");
const comment = comments.add(
	3, // (inclusive)
	8, // (exclusive): references "world"
	IntervalType.SlideOnRemove,
	{
		creator: "my-user-id",
		handle: myCommentThreadDDS.handle,
	},
);
//   content: hi world!
// positions: 012345678
//   comment:    [    )

// Interval collection supports iterating over all intervals via Symbol.iterator or `.map()`:
const allIntervalsInCollection = Array.from(comments);
const allProperties = comments.map((comment) => comment.properties);
// or iterating over intervals overlapping a region:
const intervalsOverlappingFirstHalf = comments.findOverlappingIntervals(0, 4);

// Interval endpoints are LocalReferencePositions, so all APIs in the above section can be used:
const startPosition = sharedString.localReferencePositionToPosition(comment.start); // returns 3
const endPosition = sharedString.localReferencePositionToPosition(comment.end); // returns 8: note this is exclusive!

// Intervals can be modified:
comments.change(comment.getIntervalId(), 0, 1);
//   content: hi world!
// positions: 012345678
//   comment: [)

// their properties can be changed:
comments.changeProperties(comment.getIntervalId(), { status: "resolved" });
// comment.properties === { creator: 'my-user-id', handle: <some DDS handle object>, status: "resolved" }

// and they can be removed:
comments.removeIntervalById(comment.getIntervalId());

Interval stickiness​

"Stickiness" refers to the behavior of intervals when text is inserted on either side of the interval. A "sticky" interval is one which expands to include text inserted directly adjacent to it.

A "start sticky" interval is one which expands only to include text inserted to the start of it. An "end sticky" interval is the same, but with regard to text inserted adjacent to the end.

For example, let's look at the string "abc". If we have an interval on the character "b", what happens when we insert text on either side of it? In the below diagrams, we represent an interval by putting a caret directly underneath the characters it contains.

Example​

Original string​
abc
 ^
No stickiness​
aXbYc
  ^

The interval does not expand to include the newly inserted characters X and Y.

Start stickiness​
aXbYc
 ^^
End stickiness​
aXbYc
  ^^
Full stickiness​
aXbYc
 ^^^

Concrete Implementation​

The above is a description of the abstract semantics of the concept of stickiness. In practice, this is implemented using the concept of "sides."

For a given sequence of N characters, there are 2N + 2 positions which can be referenced: the position immediately before and after each character, and two special endpoint segments denoting the position before and after the start and end of the sequence respectively.

By placing the endpoints of an interval either before or after a character, it is possible to make the endpoints inclusive or exclusive. An exclusive endpoint in a given direction implies stickiness in that direction. Whether an endpoint is inclusive or exclusive depends on both the Side and if it is the start or the end.

Given the string "ABCD":

// Refers to "BC". If any content is inserted before B or after C, this
// interval will include that content
//
// Picture:
// {start} - A[- B - C -]D - {end}
// {start} - A - B - C - D - {end}
collection.add(
	{ pos: 0, side: Side.After },
	{ pos: 3, side: Side.Before },
	IntervalType.SlideOnRemove,
);
// Equivalent to specifying the same positions and Side.Before.
// Refers to "ABC". Content inserted after C will be included in the
// interval, but content inserted before A will not.
// {start} -[A - B - C -]D - {end}
// {start} - A - B - C - D - {end}
collection.add(0, 3, IntervalType.SlideOnRemove);

In the case of the first interval shown, if text is deleted,

// Delete the character "B"
string.removeRange(1, 2);

The start point of the interval will slide to the position immediately before "C", and the same will be true.

{start} - A[- C -]D - {end}

In this case, text inserted immediately before "C" would be included in the interval.

string.insertText(1, "EFG");

With the string now being,

{start} - A[- E - F - G - C -]D - {end}

Note that the endpoint continues to remain with the associated character, except when the character is removed. When content containing endpoints is removed, After endpoints move backward and Before endpoints move forward to maintain their side value and inclusive/exclusive behavior.

Examples​

See the SharedString examples.