# SINK and SOURCE

**URL:** <https://forums.buildingsmart.org/t/sink-and-source/3245>\
**Category:** IFC\
**Created:** [January 9, 2021, 6:47pm UTC](https://forums.buildingsmart.org/t/sink-and-source/3245 "2021-01-09T18:47:54Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![bsbock](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.buildingsmart.org/bsbock/32/435_2.png) [@bsbock](https://forums.buildingsmart.org/u/bsbock)\
**Post date:** [January 9, 2021, 6:47pm UTC](https://forums.buildingsmart.org/t/sink-and-source/3245/1 "2021-01-09T18:47:54Z")

</div>

Here is the description of SINK and SOURCE from the [IfcFlowDirectionEnum](https://standards.buildingsmart.org/IFC/DEV/IFC4_3/RC1/HTML/schema/ifcsharedbldgserviceelements/lexical/ifcflowdirectionenum.htm)  
SOURCE: A flow source, where a substance flows out of the connection.  
SINK: A flow sink, where a substance flows into the connection.

But in the following picture (1), (2a), (2b) and (3) a substance would flow out of the connection (depending from which side you see it):

![SINK_OR_SOURCE_Bock](https://us1.discourse-cdn.com/flex016/uploads/buildingsmart1/original/2X/5/5f9bc8b3bd2b7ac564d5b86071ab43a6dd5a3f6e.png)

Is (1) and (2b) a SOURCE and (2a) and (3) a SINK?

---

<div class="post-metadata">

**Author:** ![hannu.lahtela](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@hannu.lahtela](https://forums.buildingsmart.org/u/hannu.lahtela)\
**Post date:** [January 11, 2021, 6:14am UTC](https://forums.buildingsmart.org/t/sink-and-source/3245/2 "2021-01-11T06:14:42Z")

</div>

IMO - vice versa. The naming is a bit confusing. I would say SINK=IN and SOURCE=OUT. The documentation should be:

- SOURCE: A flow source, where a substance flows out of the element.
- SINK: A flow sink, where a substance flows into the element.

---

<div class="post-metadata">

**Author:** ![jwouellette](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.buildingsmart.org/jwouellette/32/2140_2.png) [@jwouellette](https://forums.buildingsmart.org/u/jwouellette)\
**Post date:** [January 11, 2021, 2:46pm UTC](https://forums.buildingsmart.org/t/sink-and-source/3245/3 "2021-01-11T14:46:56Z")

</div>

@TLiebich, do you remember the logic for this one?

---

<div class="post-metadata">

**Author:** ![TLiebich](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.buildingsmart.org/tliebich/32/71_2.png) [@TLiebich](https://forums.buildingsmart.org/u/TLiebich)\
**Post date:** [January 14, 2021, 4:54pm UTC](https://forums.buildingsmart.org/t/sink-and-source/3245/4 "2021-01-14T16:54:14Z")

</div>

@hannu.lahtela is right - source is supplying - so OUT, and sink is receiving - so IN

as written in the spec: “SOURCE: A flow source, where a substance flows **OUT** of the connection.  
SINK: A flow sink, where a substance flows **IN** to the connection”

---

<div class="post-metadata">

**Author:** ![TLiebich](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.buildingsmart.org/tliebich/32/71_2.png) [@TLiebich](https://forums.buildingsmart.org/u/TLiebich)\
**Post date:** [January 14, 2021, 4:58pm UTC](https://forums.buildingsmart.org/t/sink-and-source/3245/5 "2021-01-14T16:58:51Z")

</div>

actually the spec in my view is correct (but maybe terminology-wise confusing)

- SOURCE: A flow source, where a substance flows **OUT** of the connection. – as in (2a) the flow is moving out of the connection point (the port) of the element
- SINK: A flow sink, where a substance flows **IN** to the connection. – as in (2b) the flow is moving into the connection point (the port) of the element

---

<div class="post-metadata">

**Author:** ![bsbock](https://sea2.discourse-cdn.com/flex016/user_avatar/forums.buildingsmart.org/bsbock/32/435_2.png) [@bsbock](https://forums.buildingsmart.org/u/bsbock)\
**Post date:** [January 15, 2021, 5:08pm UTC](https://forums.buildingsmart.org/t/sink-and-source/3245/6 "2021-01-15T17:08:22Z")

</div>

Thanks for clarification @TLiebich, but I also agree with @hannu.lahtela to add a hint in the documentation, to what the port is relatet (element or port-connection), e.g.:

SOURCE: A flow source, where a substance flows **out** of the connection **/ element**.  
SINK: A flow sink, where a substance flows **into** the connection **/ element**.

As a picture:

![SINK_OR_SOURCE_Bock_solved](https://us1.discourse-cdn.com/flex016/uploads/buildingsmart1/original/2X/7/7c97d66d621a9adb32ed4ec7b3c0426c98dd1616.png)