diff --git a/src/requester/instance-id.c b/src/requester/instance-id.c
new file mode 100644
index 0000000..e3cf41f
--- /dev/null
+++ b/src/requester/instance-id.c
@@ -0,0 +1,177 @@
+// NOLINTNEXTLINE(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp)
+#define _GNU_SOURCE
+#include "libpldm/requester/instance-id.h"
+#include "libpldm/pldm.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define PLDM_TID_MAX 256
+#define PLDM_INST_ID_MAX 32
+
+struct pldm_instance_db {
+	pldm_instance_id_t prev[PLDM_TID_MAX];
+	int lock_db_fd;
+};
+
+static inline int iid_next(pldm_instance_id_t cur)
+{
+	return (cur + 1) % PLDM_INST_ID_MAX;
+}
+
+int pldm_instance_db_init(struct pldm_instance_db **ctx, const char *dbpath)
+{
+	struct pldm_instance_db *l_ctx;
+
+	/* Make sure the provided pointer was initialised to NULL. In the future
+	 * if we stabilise the ABI and expose the struct definition the caller
+	 * can potentially pass a valid pointer to a struct they've allocated
+	 */
+	if (!ctx || *ctx) {
+		return -EINVAL;
+	}
+
+	l_ctx = calloc(1, sizeof(struct pldm_instance_db));
+	if (!l_ctx) {
+		return -ENOMEM;
+	}
+
+	/* Initialise previous ID values so the next one is zero */
+	for (int i = 0; i < PLDM_TID_MAX; i++) {
+		l_ctx->prev[i] = 31;
+	}
+
+	/* Lock database may be read-only, either by permissions or mountpoint
+	 */
+	l_ctx->lock_db_fd = open(dbpath, O_RDONLY | O_CLOEXEC);
+	if (l_ctx->lock_db_fd < 0) {
+		free(l_ctx);
+		return -errno;
+	}
+	*ctx = l_ctx;
+
+	return 0;
+}
+
+int pldm_instance_db_init_default(struct pldm_instance_db **ctx)
+{
+	return pldm_instance_db_init(ctx,
+				     "/usr/share/libpldm/instance-db/default");
+}
+
+int pldm_instance_db_destroy(struct pldm_instance_db *ctx)
+{
+	if (!ctx) {
+		return 0;
+	}
+	close(ctx->lock_db_fd);
+	free(ctx);
+	return 0;
+}
+
+int pldm_instance_id_alloc(struct pldm_instance_db *ctx, pldm_tid_t tid,
+			   pldm_instance_id_t *iid)
+{
+	static const struct flock cfls = {
+	    .l_type = F_RDLCK,
+	    .l_whence = SEEK_SET,
+	    .l_len = 1,
+	};
+	static const struct flock cflx = {
+	    .l_type = F_WRLCK,
+	    .l_whence = SEEK_SET,
+	    .l_len = 1,
+	};
+	uint8_t l_iid;
+
+	if (!iid) {
+		return -EINVAL;
+	}
+
+	l_iid = ctx->prev[tid];
+	if (l_iid >= PLDM_INST_ID_MAX) {
+		return -EPROTO;
+	}
+
+	while ((l_iid = iid_next(l_iid)) != ctx->prev[tid]) {
+		struct flock flop;
+		off_t loff;
+		int rc;
+
+		/* Derive the instance ID offset in the lock database */
+		loff = tid * PLDM_INST_ID_MAX + l_iid;
+
+		/* Reserving the TID's IID. Done via a shared lock */
+		flop = cfls;
+		flop.l_start = loff;
+		rc = fcntl(ctx->lock_db_fd, F_OFD_SETLK, &flop);
+		if (rc < 0) {
+			if (errno == EAGAIN || errno == EINTR) {
+				return -EAGAIN;
+			}
+			return -EPROTO;
+		}
+
+		/*
+		 * If we *may* promote the lock to exclusive then this IID is
+		 * only reserved by us. This is now our allocated IID.
+		 *
+		 * If we *may not* promote the lock to exclusive then this IID
+		 * is also reserved on another file descriptor. Move on to the
+		 * next IID index.
+		 *
+		 * Note that we cannot actually *perform* the promotion in
+		 * practice because this is prevented by the lock database being
+		 * opened O_RDONLY.
+		 */
+		flop = cflx;
+		flop.l_start = loff;
+		rc = fcntl(ctx->lock_db_fd, F_OFD_GETLK, &flop);
+		if (rc < 0) {
+			if (errno == EAGAIN || errno == EINTR) {
+				return -EAGAIN;
+			}
+			return -EPROTO;
+		}
+
+		/* F_UNLCK is the type of the lock if we could successfully
+		 * promote it to F_WRLCK */
+		if (flop.l_type == F_UNLCK) {
+			ctx->prev[tid] = l_iid;
+			*iid = l_iid;
+			return 0;
+		}
+		if (flop.l_type != F_RDLCK) {
+			return -EPROTO;
+		}
+	}
+
+	/* Failed to allocate an IID after a full loop. Make the caller try
+	 * again */
+	return -EAGAIN;
+}
+
+int pldm_instance_id_free(struct pldm_instance_db *ctx, pldm_tid_t tid,
+			  pldm_instance_id_t iid)
+{
+	static const struct flock cflu = {
+	    .l_type = F_UNLCK,
+	    .l_whence = SEEK_SET,
+	    .l_len = 1,
+	};
+	struct flock flop;
+	int rc;
+
+	flop = cflu;
+	flop.l_start = tid * PLDM_INST_ID_MAX + iid;
+	rc = fcntl(ctx->lock_db_fd, F_OFD_SETLK, &flop);
+	if (rc < 0) {
+		if (errno == EAGAIN || errno == EINTR) {
+			return -EAGAIN;
+		}
+		return -EPROTO;
+	}
+
+	return 0;
+}