92 lines
3.9 KiB
Python
92 lines
3.9 KiB
Python
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"""Making a retried request safe to retry.
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A client posts, the connection drops before the response arrives, and now nobody
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knows whether it worked. Every integration hits this eventually, and there are
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only two behaviours available: retry and risk doing it twice, or do not retry and
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risk not doing it at all. Both are wrong.
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An `Idempotency-Key` header makes the choice unnecessary. The first request runs
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and its response is kept; a repeat with the same key gets that same response back
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without the work happening again.
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Three columns carry the weight:
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**`request_hash`.** Reusing a key for a *different* request is a client bug, and
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the dangerous kind: without this, "charge £10" retried as "charge £1000" would
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quietly return the £10 response and the caller would believe the second one
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happened. Mismatched hashes are refused rather than answered.
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**`state`.** A key inserted before the work starts is what makes two simultaneous
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requests safe — the second loses the unique index and is told the first is still
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running, rather than both proceeding.
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**`expires_at`.** A retry happens within seconds or minutes. Keeping keys for ever
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would mean a client that generates them per hour eventually collides with its own
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history, and the table grows without bound for no benefit.
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Revision ID: c9f2b5d7e10d
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Revises: b8e1c4a6f90c
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"""
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from typing import Sequence, Union
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import sqlalchemy as sa
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from alembic import op
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from sqlalchemy.dialects.postgresql import JSONB, UUID
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revision: str = "c9f2b5d7e10d"
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down_revision: Union[str, Sequence[str], None] = "b8e1c4a6f90c"
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branch_labels: Union[str, Sequence[str], None] = None
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depends_on: Union[str, Sequence[str], None] = None
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def upgrade() -> None:
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op.create_table(
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"idempotency_records",
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sa.Column("id", UUID(as_uuid=True), primary_key=True,
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server_default=sa.text("gen_random_uuid()")),
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sa.Column("tenant_id", UUID(as_uuid=True),
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sa.ForeignKey("tenants.id", ondelete="CASCADE"), nullable=True),
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sa.Column("user_id", UUID(as_uuid=True),
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sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=True),
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sa.Column("idempotency_key", sa.String(255), nullable=False),
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sa.Column("endpoint", sa.String(255), nullable=False),
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sa.Column("request_hash", sa.String(64), nullable=False),
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sa.Column("state", sa.String(20), nullable=False, server_default="in_progress"),
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sa.Column("response_status", sa.Integer(), nullable=True),
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sa.Column("response_body", JSONB(), nullable=True),
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sa.Column("expires_at", sa.DateTime(timezone=True), nullable=False),
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sa.Column("created_at", sa.DateTime(timezone=True), nullable=False,
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server_default=sa.func.now()),
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sa.Column("completed_at", sa.DateTime(timezone=True), nullable=True),
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sa.UniqueConstraint("tenant_id", "user_id", "idempotency_key", "endpoint",
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name="uq_idempotency_scope"),
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)
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op.create_index(
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"ix_idempotency_expiry", "idempotency_records", ["expires_at"],
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)
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op.execute("ALTER TABLE idempotency_records ENABLE ROW LEVEL SECURITY")
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op.execute("ALTER TABLE idempotency_records FORCE ROW LEVEL SECURITY")
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op.execute(
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"""
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CREATE POLICY tenant_isolation ON idempotency_records
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USING (
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current_setting('app.bypass_rls', true) = 'on'
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OR tenant_id IS NULL
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OR tenant_id = NULLIF(current_setting('app.tenant_id', true), '')::uuid
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)
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WITH CHECK (
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current_setting('app.bypass_rls', true) = 'on'
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OR tenant_id IS NULL
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OR tenant_id = NULLIF(current_setting('app.tenant_id', true), '')::uuid
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)
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"""
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)
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def downgrade() -> None:
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op.execute("DROP POLICY IF EXISTS tenant_isolation ON idempotency_records")
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op.drop_index("ix_idempotency_expiry", table_name="idempotency_records")
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op.drop_table("idempotency_records")
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