Asynchronous Backing¶
Introduction¶
Asynchronous backing often shortened to Async Backing is a parachain protocol feature that significantly improves performance, enabling parachains to produce blocks twice as fast (every 6 seconds instead of every 12) and to provide 4x more execution time per block (2 seconds instead of 0.5).
Technically, async backing is a parachain configuration that allows collators and validators to build blocks ahead of the relay chain during the generation and backing stages of the Inclusion Pipeline by using unincluded segments, which are chains of parachain blocks that have not yet been fully included in the relay chain. This decouples parachain block production from relay chain inclusion, improves coretime efficiency, and enables the parallel processing required for parachains to further scale throughput using Elastic Scaling.
Configurations¶
The following configurations can be set by on-chain governance, dictating how many blocks ahead of the relay chain a given parachain's collators can run:
max_candidate_depth: the number of parablocks a collator can produce that are not yet included in the relay chain. A value of2means that there can be a maximum of 3 unincluded parablocks at any given time.allowed_ancestry_len: the oldest relay parent a parablock can be built on top of. A value of1means collators can start building blocks 6 seconds in advance.
Synchronous VS. Asynchronous Processing¶
The Polkadot-parachain protocol originally operated in synchronous mode, where both collators and validators drew context exclusively from the relay parent of the prior parablock, which lives on the relay chain. This made the Backing and Generation steps tightly coupled to the prior parablock completing the entire inclusion pipeline. As a result, one parablock could only be processed every other relay block, with just 0.5 seconds assigned for execution.
---
displayMode: compact
config:
themeCSS: "
#item1 { fill: #450693; stroke: #450693; } \n
#item2 { fill: #8C00FF; stroke: #8C00FF; } \n
#item3 { fill: #FFC400; stroke: #FFC400; } \n
#r { fill: #eb4172; stroke:none; font-size: 20px; } \n
svg text { font-size: 20px !important; } \n
svg .sectionTitle { font-size: 20px !important; } \n #p1padTop { display: none; } \n
/* Hide ALL task labels by default */
text.taskText,
text.taskTextOutside,
[class*='taskText'] tspan { display: none !important; } \n
/* Show labels for the 'r' group (inside or outside, incl. tspans) */
text.taskText[id^='r'],
text.taskTextOutside[id^='r'],
text[id^='r'] tspan { display: inline !important; font-size: 20px; fill: var(--white) !important; } \n
/* Keep section titles styled */
.sectionTitle { fill: var(--md-default-fg-color) !important; font-weight: 700; font-size: 18px; } \n
/* Hide the first two section titles (F1, F2). Change indexes if needed. */
.sectionTitle:nth-of-type(1),
.sectionTitle:nth-of-type(2) { display: none !important; } \n
/* Also hide SPACING row labels on the left */
text.taskTextOutside[id^='p1padTop'] { display: none !important; } \n
.grid .tick text { fill: var(--md-default-fg-color) !important; font-size: 20px !important; }
"
themeVariables:
sectionBkgColor: '#fff'
gantt:
numberSectionStyles: 1
barHeight: 70
gridLineStartPadding: 100
---
%%{init: {"gantt": {"barHeight": 70 }}}%%
gantt
dateFormat YYYY
axisFormat %y
tickInterval '10year'
section F1
R1 : r, 1905, 1907
R2 : r, 1911, 1913
R3 : r, 1917, 1919
R4 : r, 1923, 1925
section F2
SPACING : p1padTop, 1901, 1924
section P1
X : item1, 1900, 1901
Backing : item2, 1901, 1906
Inclusion : item3, 1906, 1912
section P2
X : item1, 1912, 1913
Backing : item2, 1913, 1918
Inclusion : item3, 1918, 1924
The modern protocol now uses asynchronous backing, where both collators and validators have access to unincluded segments as an additional context source. The Backing and Generation steps are no longer coupled to the prior block completing the full inclusion pipeline. Instead, the prior parablock only needs to complete the generation step and be added to the Unincluded Segments before the next parablock can begin the Backing and Generation steps.
This results in one parablock being processed every relay block (instead of every other relay block), and allows for more time to execute during the Generation step (0.5s → 2s).
---
displayMode: compact
config:
themeCSS: "
#item1 { fill: #450693; stroke: #450693; } \n
#item2 { fill: #8C00FF; stroke: #8C00FF; } \n
#item3 { fill: #FFC400; stroke: #FFC400; } \n
#r { fill: #eb4172; stroke:none; font-size: 20px; } \n
svg text { font-size: 20px !important; } \n
svg .sectionTitle { font-size: 20px !important; } \n #p1padTop { display: none; } \n
/* Hide ALL task labels by default */
text.taskText,
text.taskTextOutside,
[class*='taskText'] tspan { display: none !important; } \n
/* Show labels for the 'r' group (inside or outside, incl. tspans) */
text.taskText[id^='r'],
text.taskTextOutside[id^='r'],
text[id^='r'] tspan { display: inline !important; font-size: 20px; color: var(--white) !important; } \n
/* Keep section titles styled */
.sectionTitle { fill: var(--md-default-fg-color) !important; font-weight: 700; font-size: 18px; } \n
/* Hide the first two section titles (F1, F2). Change indexes if needed. */
.sectionTitle:nth-of-type(1),
.sectionTitle:nth-of-type(2) { display: none !important; } \n
/* Also hide SPACING row labels on the left */
text.taskTextOutside[id^='p1padTop'] { display: none !important; } \n
.taskTextOutsideRight { fill: var(--md-default-fg-color) !important; font-size: 20px !important; }
.grid .tick text { fill: var(--md-default-fg-color) !important; font-size: 20px !important; }
"
themeVariables:
sectionBkgColor: '#fff'
gantt:
numberSectionStyles: 1
barHeight: 70
gridLineStartPadding: 100
---
%%{init: {"gantt": {"barHeight": 70 }}}%%
gantt
dateFormat YYYY
axisFormat %y
tickInterval '10year'
section F1
R1 : r, 1905, 1907
R2 : r, 1911, 1913
R3 : r, 1917, 1919
R4 : r, 1923, 1925
R5 : r, 1929, 1931
section F2
SPACING : p1padTop, 1901, 1930
section P1
X : item1, 1900, 1902
Backing : item2, 1902, 1912
Inclusion : item3, 1912, 1918
section P2
X : item1, 1906, 1908
Backing : item2, 1908, 1918
Inclusion : item3, 1918, 1924
section P3
X : item1, 1912, 1914
Backing : item2, 1914, 1924f
Inclusion : item3, 1924, 1930
section P4
X : item1, 1918, 1920
Backing : item2, 1920, 1930| Created: January 14, 2026