diff --git a/.vscode/settings.json b/.vscode/settings.json
index 48fb5ea..8870c9a 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -1,5 +1,5 @@
 {
-    "rust-analyzer.checkOnSave.allTargets": false,
+    "rust-analyzer.checkOnSave.allTargets": true,
     "rust-analyzer.cargo.features": ["log"],
     "rust-analyzer.cargo.target": "x86_64-unknown-linux-gnu"
 }
\ No newline at end of file
diff --git a/src/defender_metrics/aws_types.rs b/src/defender_metrics/aws_types.rs
new file mode 100644
index 0000000..903254d
--- /dev/null
+++ b/src/defender_metrics/aws_types.rs
@@ -0,0 +1,84 @@
+use core::str::FromStr;
+
+use heapless::{LinearMap, String, Vec};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct TcpConnections {
+    #[serde(rename = "ec")]
+    pub established_connections: Option<EstablishedConnections>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct EstablishedConnections {
+    #[serde(rename = "cs")]
+    pub connections: Option<Vec<Connection, { MAX_CONNECTIONS }>>,
+
+    #[serde(rename = "t")]
+    pub total: Option<u64>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Connection {
+    #[serde(rename = "rad")]
+    pub remote_addr: String<REMOTE_ADDR_SIZE>,
+
+    /// Port number, must be >= 0
+    #[serde(rename = "lp")]
+    pub local_port: Option<u16>,
+
+    /// Interface name
+    #[serde(rename = "li")]
+    pub local_interface: Option<String<LOCAL_INTERFACE_SIZE>>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ListeningTcpPorts {
+    #[serde(rename = "pts")]
+    pub ports: Option<Vec<TcpPort, MAX_PORTS>>,
+
+    #[serde(rename = "t")]
+    pub total: Option<u64>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct TcpPort {
+    #[serde(rename = "pt")]
+    pub port: u16,
+
+    #[serde(rename = "if")]
+    pub interface: Option<String<LOCAL_INTERFACE_SIZE>>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ListeningUdpPorts {
+    #[serde(rename = "pts")]
+    pub ports: Option<Vec<UdpPort, MAX_PORTS>>,
+
+    #[serde(rename = "t")]
+    pub total: Option<u64>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct UdpPort {
+    #[serde(rename = "pt")]
+    pub port: u16,
+
+    #[serde(rename = "if")]
+    pub interface: Option<String<LOCAL_INTERFACE_SIZE>>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct NetworkStats {
+    #[serde(rename = "bi")]
+    pub bytes_in: Option<u64>,
+
+    #[serde(rename = "bo")]
+    pub bytes_out: Option<u64>,
+
+    #[serde(rename = "pi")]
+    pub packets_in: Option<u64>,
+
+    #[serde(rename = "po")]
+    pub packets_out: Option<u64>,
+}
diff --git a/src/defender_metrics/data_types.rs b/src/defender_metrics/data_types.rs
new file mode 100644
index 0000000..8d3dab1
--- /dev/null
+++ b/src/defender_metrics/data_types.rs
@@ -0,0 +1,120 @@
+use core::str::FromStr;
+
+use heapless::{LinearMap, String, Vec};
+use serde::{Deserialize, Serialize};
+
+// Constants for heapless container sizes
+pub const HEADER_VERSION_SIZE: usize = 6;
+pub const REMOTE_ADDR_SIZE: usize = 64;
+pub const LOCAL_INTERFACE_SIZE: usize = 32;
+pub const MAX_METRICS: usize = 8;
+pub const MAX_CUSTOM_METRICS: usize = 16;
+pub const MAX_CUSTOM_METRICS_NAME: usize = 32;
+
+pub enum MetricError {
+    Malformed,
+    Throttled,
+    MissingHeader,
+    Other,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Metric {
+    #[serde(rename = "hed")]
+    pub header: Header,
+
+    #[serde(rename = "cmet")]
+    pub custom_metrics: Option<
+        LinearMap<String<MAX_CUSTOM_METRICS_NAME>, Vec<CustomMetric, 1>, MAX_CUSTOM_METRICS>,
+    >,
+}
+
+impl Metric {
+    pub fn new(
+        custom_metrics: Option<
+            LinearMap<String<MAX_CUSTOM_METRICS_NAME>, Vec<CustomMetric, 1>, MAX_CUSTOM_METRICS>,
+        >,
+        timestamp: i64,
+    ) -> Self {
+        let header = Header {
+            report_id: timestamp,
+            version: String::<HEADER_VERSION_SIZE>::from_str("1.0").unwrap(), //FIXME: Don't
+        };
+
+        Self {
+            header,
+            custom_metrics,
+        }
+    }
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Header {
+    /// Monotonically increasing value. Epoch timestamp recommended.
+    #[serde(rename = "rid")]
+    pub report_id: i64,
+
+    /// Version in Major.Minor format.
+    #[serde(rename = "v")]
+    pub version: String<HEADER_VERSION_SIZE>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum CustomMetric {
+    Number(u64),
+    NumberList(Vec<u64, MAX_METRICS>),
+    StringList(Vec<String<LOCAL_INTERFACE_SIZE>, MAX_METRICS>),
+    IpList(Vec<String<REMOTE_ADDR_SIZE>, MAX_METRICS>),
+}
+
+impl CustomMetric {
+    pub fn new_number(value: u64) -> heapless::Vec<Self, 1> {
+        let mut custom_metric_map = Vec::new();
+
+        custom_metric_map.push(CustomMetric::Number(value)).unwrap();
+
+        custom_metric_map
+    }
+
+    pub fn new_number_list(values: &[u64]) -> heapless::Vec<Self, 1> {
+        let mut custom_metric_map = Vec::new();
+
+        let mut vec = Vec::new();
+        for &v in values {
+            vec.push(v).unwrap();
+        }
+
+        custom_metric_map
+            .push(CustomMetric::NumberList(vec))
+            .unwrap();
+
+        custom_metric_map
+    }
+
+    pub fn new_string_list(values: &[&str]) -> heapless::Vec<Self, 1> {
+        let mut custom_metric_map = Vec::new();
+
+        let mut vec = Vec::new();
+        for &v in values {
+            vec.push(String::from_str(v).unwrap()).unwrap();
+        }
+        custom_metric_map
+            .push(CustomMetric::StringList(vec))
+            .unwrap();
+
+        custom_metric_map
+    }
+
+    pub fn new_ip_list(values: &[&str]) -> heapless::Vec<Self, 1> {
+        let mut custom_metric_map = Vec::new();
+
+        let mut vec = Vec::new();
+        for &v in values {
+            vec.push(String::from_str(v).unwrap()).unwrap();
+        }
+        custom_metric_map.push(CustomMetric::IpList(vec)).unwrap();
+
+        custom_metric_map
+    }
+}
diff --git a/src/defender_metrics/mod.rs b/src/defender_metrics/mod.rs
index afe4e4d..1789f17 100644
--- a/src/defender_metrics/mod.rs
+++ b/src/defender_metrics/mod.rs
@@ -1,9 +1,17 @@
+use data_types::{
+    CustomMetric, Metric, MetricError, HEADER_VERSION_SIZE, MAX_CUSTOM_METRICS,
+    MAX_CUSTOM_METRICS_NAME,
+};
 use embassy_sync::blocking_mutex::raw::RawMutex;
-use embedded_mqtt::{Publish, Subscribe, SubscribeTopic, ToPayload};
+use embedded_mqtt::{DeferredPayload, Publish, Subscribe, SubscribeTopic, ToPayload};
+use futures::StreamExt;
+use heapless::LinearMap;
 use topics::Topic;
 
 use crate::shadows::Error;
 
+// pub mod aws_types;
+pub mod data_types;
 pub mod topics;
 
 pub struct MetricHandler<'a, 'm, M: RawMutex> {
@@ -11,6 +19,45 @@ pub struct MetricHandler<'a, 'm, M: RawMutex> {
 }
 
 impl<'a, 'm, M: RawMutex> MetricHandler<'a, 'm, M> {
+    pub fn new(mqtt: &'m embedded_mqtt::MqttClient<'a, M>) -> Self {
+        Self { mqtt }
+    }
+
+    pub async fn publish_metric(
+        &self,
+        custom_metric: LinearMap<
+            heapless::String<{ MAX_CUSTOM_METRICS_NAME }>,
+            heapless::Vec<CustomMetric, 1>,
+            { MAX_CUSTOM_METRICS },
+        >,
+        metric_name: &str,
+        timestamp: i64,
+    ) -> Result<(), MetricError> {
+        let metric = Metric::new(Some(custom_metric), timestamp);
+
+        let payload = DeferredPayload::new(
+            |buf: &mut [u8]| {
+                serde_json_core::to_slice(&metric, buf)
+                    .map_err(|_| embedded_mqtt::EncodingError::BufferSize)
+            },
+            4000,
+        );
+
+        let mut subsctiption = self
+            .publish_and_subscribe(payload, metric_name)
+            .await
+            .map_err(|_| MetricError::Other)?;
+
+        loop {
+            let message = subsctiption.next().await.ok_or(MetricError::Malformed)?;
+
+            match Topic::from_str(message.topic_name()) {
+                Some(Topic::Accepted) => return Ok(()),
+                Some(Topic::Rejected) => return Err(MetricError::Other),
+                _ => (),
+            };
+        }
+    }
     async fn publish_and_subscribe(
         &self,
         payload: impl ToPayload,
@@ -56,3 +103,116 @@ impl<'a, 'm, M: RawMutex> MetricHandler<'a, 'm, M> {
         Ok(sub)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use core::str::FromStr;
+
+    use super::data_types::*;
+
+    use heapless::{LinearMap, String};
+
+    #[test]
+    fn number() {
+        let mut custom_metrics = LinearMap::new();
+
+        let name_of_metric = String::from_str("myMetric").unwrap();
+
+        custom_metrics
+            .insert(name_of_metric, CustomMetric::new_number(123))
+            .unwrap();
+
+        let metric = Metric::new(Some(custom_metrics), 123123123123);
+
+        let payload: String<4000> = serde_json_core::to_string(&metric).unwrap();
+
+        println!("buffer = {}", payload);
+
+        assert!(true)
+    }
+
+    #[test]
+    fn number_list() {
+        let mut custom_metrics = LinearMap::new();
+
+        // NUMBER LIST
+        let my_number_list = String::from_str("my_number_list").unwrap();
+
+        custom_metrics
+            .insert(
+                my_number_list,
+                CustomMetric::new_number_list(&[123, 456, 789]),
+            )
+            .unwrap();
+
+        let metric = Metric::new(Some(custom_metrics), 123123123123);
+
+        let payload: String<4000> = serde_json_core::to_string(&metric).unwrap();
+
+        println!("buffer = {}", payload);
+
+        assert!(true)
+    }
+
+    #[test]
+    fn string_list() {
+        let mut custom_metrics = LinearMap::new();
+
+        // STRING LIST
+        let my_string_list = String::from_str("my_string_list").unwrap();
+
+        custom_metrics
+            .insert(
+                my_string_list,
+                CustomMetric::new_string_list(&["value_1", "value_2"]),
+            )
+            .unwrap();
+
+        let metric = Metric::new(Some(custom_metrics), 123123123123);
+
+        let payload: String<4000> = serde_json_core::to_string(&metric).unwrap();
+
+        println!("buffer = {}", payload);
+
+        assert!(true)
+    }
+
+    #[test]
+    fn all_types() {
+        let mut custom_metrics = LinearMap::new();
+
+        let my_number = String::from_str("MyMetricOfType_Number").unwrap();
+        custom_metrics
+            .insert(my_number, CustomMetric::new_number(1))
+            .unwrap();
+
+        let my_number_list = String::from_str("MyMetricOfType_NumberList").unwrap();
+        custom_metrics
+            .insert(my_number_list, CustomMetric::new_number_list(&[1, 2, 3]))
+            .unwrap();
+
+        let my_string_list = String::from_str("MyMetricOfType_StringList").unwrap();
+        custom_metrics
+            .insert(
+                my_string_list,
+                CustomMetric::new_string_list(&["value_1", "value_2"]),
+            )
+            .unwrap();
+
+        let my_ip_list = String::from_str("MyMetricOfType_IpList").unwrap();
+        custom_metrics
+            .insert(
+                my_ip_list,
+                CustomMetric::new_ip_list(&["172.0.0.0", "172.0.0.10"]),
+            )
+            .unwrap();
+
+        let metric = Metric::new(Some(custom_metrics), 123123123123);
+
+        let payload: String<4000> = serde_json_core::to_string(&metric).unwrap();
+
+        println!("buffer = {}", payload);
+
+        assert!(true)
+    }
+}
diff --git a/src/defender_metrics/topics.rs b/src/defender_metrics/topics.rs
index 41d545a..bca9e66 100644
--- a/src/defender_metrics/topics.rs
+++ b/src/defender_metrics/topics.rs
@@ -17,7 +17,7 @@ impl Topic {
     const PREFIX: &'static str = "$aws/things";
     const NAME: &'static str = "defender/metrics";
     //TODO: Feature gate json or cbor
-    const PAYLOAD_FORMAT: &'static str = "cbor";
+    const PAYLOAD_FORMAT: &'static str = "json";
 
     pub fn format<const L: usize>(
         &self,
@@ -55,6 +55,22 @@ impl Topic {
 
         Ok(topic_path)
     }
+
+    pub fn from_str(s: &str) -> Option<Topic> {
+        let tt = s.splitn(7, '/').collect::<heapless::Vec<&str, 7>>();
+        match (tt.get(0), tt.get(1), tt.get(3), tt.get(4)) {
+            (Some(&"$aws"), Some(&"things"), Some(&"defender"), Some(&"metrics")) => {
+                // This is a defender metric topic, now figure out which one.
+
+                match tt.get(6) {
+                    Some(&"accepted") => Some(Topic::Accepted),
+                    Some(&"rejected") => Some(Topic::Rejected),
+                    _ => return None,
+                }
+            }
+            _ => None,
+        }
+    }
 }
 
 #[derive(Default)]
diff --git a/src/shadows/mod.rs b/src/shadows/mod.rs
index 9c728f2..958637f 100644
--- a/src/shadows/mod.rs
+++ b/src/shadows/mod.rs
@@ -123,7 +123,7 @@ where
         };
 
         let payload = DeferredPayload::new(
-            |buf| {
+            |buf: &mut [u8]| {
                 serde_json_core::to_slice(&request, buf)
                     .map_err(|_| embedded_mqtt::EncodingError::BufferSize)
             },