How to Reconstruct a Scope's State at a Point in Time¶

This guide shows you how to rebuild what a scope believed at a past moment using get_scope_at: structural time-travel computed from chain ordering, with no stored snapshots.

The key distinction to keep in mind: get_scope_at rewinds beliefs to the value they held at the cut, whereas query_scope's event range drops beliefs outside the range. This guide makes that difference concrete.

Requirements¶

A constructed MemoryCore with a belief that has been revised over time. See Your First Belief Store.

Call get_scope_at with an as_of event id¶

get_scope_at takes a scope id and an as_of_event_id (a UUID7 cut point) and returns the active belief base as it stood at that cut. The cut point is typically a source_event_id you captured at an earlier write.

from uuid import uuid7

from doxastica import InMemoryBackend, MemoryCore

core = MemoryCore(InMemoryBackend())
scope = "mission-control"

core.revise(scope, "satellite-status", "nominal", source_event_id=uuid7())

cut = uuid7()
core.revise(scope, "satellite-status", "degraded", source_event_id=cut)

# The status changes again, after the cut.
core.revise(scope, "satellite-status", "recovering", source_event_id=uuid7())

# Reconstruct the base as of the cut.
past = core.get_scope_at(scope, as_of_event_id=cut)
print({b.belief_id: b.value for b in past})

You should see the value that was current at the cut, not the latest one:

{'satellite-status': 'degraded'}

The belief rewound to "degraded" (the value it held at cut), and the later "recovering" revision is excluded.

Inclusive-cut semantics¶

The cut is inclusive: a state whose source_event_id equals as_of_event_id is included. This is what makes reconstructing at the latest event id give you exactly the current base.

from doxastica import BeliefFilter

latest_event = uuid7()
core.revise(scope, "satellite-status", "nominal", source_event_id=latest_event)

reconstructed = core.get_scope_at(scope, as_of_event_id=latest_event)
current = core.query_scope(scope, BeliefFilter())

print({b.belief_id: b.value for b in reconstructed})  # {'satellite-status': 'nominal'}
print({b.belief_id: b.value for b in current})  # {'satellite-status': 'nominal'}

A belief that was retracted at or before the cut is absent from the reconstruction, exactly as it would be in the current base: the cut applies the same active-tail rule, just over the window up to as_of.

Why no include_retracted here

get_scope_at has no include_retracted flag. It always reconstructs the active base as of the cut; that is the meaning of "what did this scope believe then." To browse retracted states, use get_revision_chain (see How to Inspect Revision History).

Contrast with query_scope's event range¶

It is easy to confuse get_scope_at(scope, as_of) with query_scope(scope, BeliefFilter(event_id_max=as_of)). They are different operations:

	`get_scope_at(scope, as_of)`	`query_scope(scope, BeliefFilter(event_id_max=as_of))`
Cut applied	before picking each belief's latest state	after picking each belief's current tail
Effect on a since-revised belief	rewinds to the value held at the cut	drops the belief entirely
Use when	You want the past state of the scope	You want current beliefs whose latest write is `<= as_of`

The example below shows the divergence directly. The belief's latest write is after the cut:

core = MemoryCore(InMemoryBackend())
scope = "mission-control"

cut = uuid7()
core.revise(scope, "satellite-status", "degraded", source_event_id=cut)
core.revise(scope, "satellite-status", "recovering", source_event_id=uuid7())

# get_scope_at REWINDS to the value at the cut:
print(core.get_scope_at(scope, as_of_event_id=cut))
# [BeliefState(... value='degraded' ...)]

# query_scope event_id_max DROPS the belief (its current tail is newer than the cut):
from doxastica import BeliefFilter

print(core.query_scope(scope, BeliefFilter(event_id_max=cut)))
# []

get_scope_at returns the rewound "degraded" state; query_scope returns nothing, because the belief's current tail is "recovering", which is newer than the bound. For the query_scope side, see How to Query the Current Belief Base with BeliefFilter.

Verification¶

core = MemoryCore(InMemoryBackend())
scope = "mission-control"

core.revise(scope, "satellite-status", "nominal", source_event_id=uuid7())
cut = uuid7()
core.revise(scope, "satellite-status", "degraded", source_event_id=cut)
core.revise(scope, "satellite-status", "recovering", source_event_id=uuid7())

past = core.get_scope_at(scope, as_of_event_id=cut)
assert {b.value for b in past} == {"degraded"}  # rewound, inclusive of the cut

Getting "degraded" (the value at the cut, not the latest "recovering") confirms inclusive rewind semantics.